britbox
Multiple Major Winner
I just find it refreshing that even you aren't exempt from Ricardo's mockery (his "wheelchair fan" comment implies you, as you're the only one suggesting he wouldn't have been able to keep up).
But seriously, most seem to have settled in the 6-7 range. I think Ricardo's 8-9 is based upon the hypothetical that he would have aged more like Federer, as opposed to Wilander. I personally think something more in-between would have been likely, considering that both Federer and Wilander are outliers to the norm. Fed's last Slam (so far) being at age 36, and Mats being at age 24.
Here are the ages at which every 6+ Slam winner ("all-time great") of the Open Era won their last Slam (includes all 6+ Slam winners who won their last Slams in the Open Era):
(Active in bold)
24, 24, 25, 27, 28, 29, 30, 31, 31, 31, 31, 32, 32, 36, 37
(That would be Borg, Wilander, McEnroe, Edberg, Becker, Lendl, Newcombe, Laver, Connors, Sampras, Djokovic, Agassi, Nadal, Federer, Rosewall).
The median age is 31, the average is 28.5.
Now obviously every player is different, and different eras yield different degrees of longevity. Players generally maintained prime form later in the 70s and more recently, with younger aging curves in the 80s to early 00s. But that at least gives us a sense of how to approach "The Borg Question." I would suggest that ages 24-25 (comprised of Wilander, Borg, and McEnroe) as well as 36-37 (Federer, Rosewall) are outliers, and the Open Era norm is age 27-32. On the other hand, it is worth noting that all of the young outliers are close to Borg's era, so maybe they need to be factored in. But cutting them out, the median of that range is also 31, but the average is 30.2. That is a good over/under age for when an all-time great "should" win his last Slam in the Open Era.
Perhaps it would be more accurate to imagine Borg's range of outcomes as Wilander to Connors. Wilander means he wouldn't have won any more Slams - that seems terribly pessimistic, but I suppose is possible (although my initial post suggest we approach the question as if he came back refreshed in 1982, when he was age 25-26). Connors means he would have won his last at age 31, so in 1987-88. Optimistic but also possible.
Of course we'll never know, but that is part of the allure of Bjorn Borg. No one knows whether he should be grouped with the best of the best (Laver, Federer, Nadal, Djokovic, Sampras), or more in the next group down with Connors, Lendl, and Mac. His peak suggests the former, while his overall career resume suggests the latter. Maybe framing it that way suggests the obvious: he's in-between.
I'm not even thinking in terms of "keeping it up"... projections at some tournaments propel him way beyond what he was doing at his peak.
Borg never used to play the Australian Open... but somehow we're guessing that if he didn't retire (mainly through burnout to be specific) then he's suddenly going to start giving up his offseason (the AO used to be played in December), pack his bags and head down under every year... and then automatically assume he's going to be winning there with his eyes closed.
I'm not buying he goes... and if he did then nothing is automatic.
Somebody was telling me once that Edberg wouldn't have won the 85 AO if Mac was there... I had to point that actually... McEnroe was there. Mac went there around his peak a couple of times and didn't win it.
I find it unlikely that Borg would play the AO if he stayed "unretired"... he did indicate he would play it if a calendar grand slam was on the cards. Based on some projections, that becomes be an annual opportunity, just not mine.
No reason to think Borg wouldn't have adapted? There are a couple at least... On his comeback he chose not to "adapt" and stuck with a wooden racquet. He did adapt with his coach though, he swapped out Lennart Bergelin for a karate expert. I wouldn't really think of either in positive terms.
I think Borg's best chances to add to his tally would always be at the FO but more generally, I'm thinking along Federberg's line that racquet technology evolving was more of an equalizer for the likes of Mac and Borg than a huge plus.