@Backhand_DTL - how do you personally regard seasons of yesteryear... like Laver's 69, Connors 74 and Mac's 84 in respect to the present game?
I didn't see any of those because I wasn't even born then, so I can basically just assess numbers. So in terms of those 1969 would be the best season of the open era because it was the only one where someone one all 4 slams. 1984 would be clearly the best two slam season and 1974 would be somewhat similar to 2006 and 2015.
But a big problem is that the tournament structure was different then, so it's hard to compare results away from the majors and the majors themselves apart from Wimbledon didn't have the same importance they have since the 90s as I think Connors and McEnroe skipped the French Open, respectively the Australian Open more or less voluntarily. That's why those years could be even better in terms of greatness than the pure numbers show and it's not possible for me to sensibly compare those to 2006, 2011 and 2015.
In that case why isn't Karlovic the number 1 ranked player in the world. I'm sorry, as much as you might want to try to use reductive logic to argue this point I just don't see it. The decline in Federer's movement and consequentially his forehand is not a small thing. It's huge. The guy's forehand in his prime is possibly the most lethal weapon the sport has ever seen. Here's a problem with stats, if you're looking at Rogers serving stats in isolation I think you're missing a lot. Federer's greatest combo was the serve and forehand combined. It was always a lethal one-two. Yes sure he always got a lot of aces, but now he's practically forced to go down the Karlovic route. What made him so difficult in the past was the forehand shot following a successful return. The likes of Novak don't even need to worry about that anymore. They know if they get it in, it's very likely a rally will ensue. Back in his prime you had to get the ball in and ensure the return was positioned away from a lethal forehand response. You simply cannot look at single shots in isolation, elite tennis is not that simple. The idea that in effect you think there has been a slight decline that's been largely compensated for by improvements elsewhere is a complete misreading of what's happened. Roger has had to improve his serve and tactics to try to hang in there. That's the best we can say
Because Karlovic is so much worse than the top players in everything but serve, volleys and backhand slice that even his superior serve cannot compensate that. But compared to himself as an overall tennis player I don't think he is any worse now with 37 than he was at 27.
In general the differences in tennis are really small as even in a win with a routine scoreline like 6:3 6:3 the ratio of points won is typically something like 55 % to 45 % which is not a big difference on absolute terms and even top 10 players sometimes win just above 50% of the points they played in a complete season. So even a slight decline or an improvement that leads to a change of 1 or 2 % can make a significant difference in results. But it's also possible that a decline in certain areas influences the way points are lost and won more than the numbers themselves. So it is completely possible that Roger wins less points with his first or second shot after the serve now but having more unreturned serves and still winning a majority of the longer rallies that happen on his serve ultimately result in him being as good at holding serve as ever.
On slower Hard Courts and clay which make the majority of the tour his decline in movement, return and on the point ending forehand definitely visibly hurt him and greatly lessen his chances against the players with elite defensive skills as well as making him beatable for a lot more players than when he was younger, but on faster courts he could adapt his game in a way that over best-of-three he was still the best player in the world in 2014 and 2015 but fell just short of beating Novak over best-of-five.