As you know, I tend to give (and still give) Novak the singular GOAT crown if I have to pick just one (gun to head), or at least give him a tiny sliver of an edge as "first among equals." But my preference is otherwise - that taking all factors into account, history is best served by seeing them as a "three-headed GOAT" - or, better yet, jettison the whole idea of GOAT.
But I'm going to put on my Fedfan hat for a moment - not to argue that he deserves the singular GOAT, but to defend this idea that he "should have retired" in 2012 or 2017, or at any given point.
For one, even after 2012, Roger was still one of the three best players for another six or seven years, at least most of the time. He struggled at times with injury, but when healthy he was right there - and still going deep in Slams, including three Finals in 2014-15, and a last one in 2019. To say he should have retired in 2012 after Wimbledon, well, it fits the Sampras mold, but part of the legend of Roger (and the Big Three as a whole)--and what sets them apart from Sampras and others--is that they came back, and they returned to the top when it seemed like they were done. And of course by suggesting that he "should have" retired after Wimbledon in 2017 ignores his 2018 AO victory, or the fact that he reached--and almost won--the 2019 Wimbledon.
My sense is that you like the narrative of going out on top - or at least after a big victory. And yeah, it is a nice one. Who knows, maybe if he had won that 2019 Wimbledon he would have retired, and it would have been a beautiful moment. Instead we got the limping anguish of 2020-22. But I don't think that tarnishes his legacy at all. Sampras' retirement is singular, as is Borg's. Everyone else retired in a more conventional way: decline and eventual retirement. Such is life (and tennis).
Now let's talk about Slam totals. Some folks (like
@Kieran in his post just above) like to focus on Roger getting early "easy" Slams, but let's also consider the second half of his career. Let's start with the hypothesis that the three of them were, at their best, basically equals. They all have their specializations, and we can argue endlessly about specifics (e.g. peak Novak vs. peak Roger on hards, how much to weigh Rafa's unparalleled clay dominance vs. his weaker resume on hards and grass, etc), but let's just assume, for a moment, that they were all--at their best, and overall as players--equally (or at least similarly) great. To use a phrase from political elections, "too close to call." They all had periods of utter dominance, all enjoyed great longevity, all areas where they were unparalleled, etc etc.
This view implies that a reliance on raw statistics, especially Slam totals, is hugely over-simplistic. Or, at the very least, if you're going to rely on statistics, look at all of them - and put them in context, rather than taking the easy route of "Slam reductionism."
I mean, it is baked into all of us: to think in terms of Slam titles as the singular measure of greatness, or at least by far the most important. Now it
may be the most important, at least in terms of bragging rights, but it shouldn't be looked at alone or aside from everything else. Furthermore, we have to consider context. I've often talked about Andy Murray as a lesser great who would have been an ATG if he played 10 years before or 10 years after. The fact is, Andy reached 11 Slam finals - the same as McEnroe, Wilander, and Edberg, and one more than Becker. He just had to face three guys who were better than any player that those four ever had to play, with the possible exception of Borg and Sampras at the tail-end and start of their careers, respectively (this is not to bag on those four guys - the mid-to-late 80s was arguably the most difficult context to win a Slam in, with more ATGs in or near their primes than in any other part of the Open Era, imo...but this is just more fuel for that notion of context being so important).
So let's consider Roger's later career. By 2011, he had not one but two guys in their primes who were not only equally talented, but significantly younger. Between the two of them, they covered the entire spectrum of the tour. And by 2011, together were his equals or betters on every court (though I'd probably give Roger the edge over Novak on grass through 2012, and maybe Roger would keep the edge on fast hards a bit longer, but that's a quibble). While we could argue that Roger was still at or near peak form for at least a good portion of his 30s, he was still in his 30s - and more so, 5-6 years older than two equally skilled players.
Now let's imagine that a 4th guy came into the mix, one born in 1992 - and thus 5-6 years younger than Novak and Rafa. Just as good as the Big Three, but 5-6 years younger than Nadalkovic. Or better yet, imagine
two such players emerging. These guys would have reached peak form sometime in the 2012-15 range and still close to peak form today, and provided huge roadblocks for the other three - but most especially Rafa and Novak (in terms of raw titles and Slam count). By the end of 2012, Roger had 17 Slams, Rafa 11, Novak 5. With two new, and younger, guys of similar ability starting to peak around then, there's no way those three get to 20+. Imagining two such players gives us a sense of the context of Roger's career, from 2012 on.
My point being, not only should we not fault Roger for eeking it out to get 18-20, we also need to put Rafa's and Novak's Slam totals in context. A large percentage of their Slams occurred during a time when neither of the next two generations (Lost Gen and Next Gen) produced any players who were even vaguely comparable, or even true All-Time Greats. The first such player that seems a surefire ATG was born in 2003 - 16 years after Novak! Meaning, we're possibly looking at a gap of
fifteen years (1988-2002) without any all-time greats being born. Even if Sinner (2001) or FAA (2000) get there, we're still talking 12+ years. And even if Daniil Medvedev resurges and improbably wins 5 more Slams to claim his place among ATGs, we're talking about a pretty significant gap (1987 to 1996).
Of course it should also be said that part of the weakness of those generations is due to the greatness of the Big Three, especially Novak and Rafa. Their greatness and longevity was like a massive concrete wall that younger players couldn't get by. But not all of it, and maybe not even most of it. I don't see any player born from 1988-2002 that I would say would have definitely or even likely been an ATG if the Big Three weren't around, or even just declined at historically typical rates. It is reciprocal, and neither Rafa nor Novak ever had to deal with anything like Roger did: two equally talented players that were half a decade or more younger.
I am
not arguing that Roger was greater than the other two. I
am arguing that raw Slam totals only tell part of the picture, and that the context of the last decade has to be kept in mind. There's a valid argument to be made that Roger's Slam total was padded early on by some weak opposition, but if we're going to do that, we also have to look at the context of the last decade, the utter lack of new all-time great challengers for Rafa and Novak, that have in turn padded their totals - would argue at least as much as Roger's early Slams.
TLDR: It is a bit silly to argue that Roger "should have" retired at any given point, whether after 2012 or after 2017. He retired when he felt he no longer could compete at the level he wanted to, and there's no shame in the last decade of his career. As far as Slam totals are concerned, not only are they part (if a major part) of the overall statistical picture of any given player, but context is hugely important: in this case, the lack of not even a single truly great player born after Novak in 1987 for possibly as much as 16 years.
machine in 2011!? 
1 slam wonder 
So there will aways be ways to slice it differently.)
It's easy to talk trash about Rafa's injury problems when you admit your player has been one of the luckiest players around. Who knows what would have happened if Rafa didn't have as many injuries? Who knows what might have happened if Novak had chronic injury issues. In 2022 we have heard every day how hard done by Novax was for missing 2 slams due to personal health choices. But if a Rafa fan brings up Rafa has missed 10+ slams due to injury we get 
Sorry but Nadal having multiple decimas 14 FO, 12 Barcelona, 11 Monte Carlo and 10 Rome is the ultimate achievement and the one record that will never be broken... True excellence, a GOD MODE level that Pushovic can only dream of. 
