OK, to the bolded part, that clarifies your argument for me. Yes, I can agree that some of the older champions were arguably equal to the best athletes among the modern champions. No problem with that first statement.
Now, if I understand correctly, you argue that Rosewall, Gonzalez and McEnroe have all lasted into old age because their styles were economical, or less hard on the body, in one way or another. That I also don't take issue with. They all did have economical styles, and all used brains rather than brawn to get their living.
Federer is very similar, and as Urban pointed out, we have already seen some good longevity from him. He won Wimbledon and regained the #1 ranking at the age of 31. Laver won his last Slam event having just turned 31 too; and as you know that was the last year that he was the unquestioned #1. Of course Laver swept all 4 Slams at that age, but he soon dropped off sharply in the majors; and if Federer wins another major it will become very arguable that he has shown greater longevity than Laver. I'm not insisting on such an argument: what I am insisting on is that Federer is already, at worst, approaching levels of longevity achieved by Laver. There's not some wide chasm between the two champions on that score.
Earlier this year Federer was a 31-year-old ranked #1 over three major rivals all younger than him: all 25 or 26 years of age. And he won Wimbledon by beating two of them: Djokovic who was ranked #1 then and was defending Wimbledon champion; Murray who was just weeks away from winning what you regard as a major, and who played very well in the Wimbledon final (almost as well as he did in the Olympic final). Already this is greater longevity than Sampras, who had a superficially similar drought in the Slams before winning his last one: but Pete won no tournaments at all during that drought, unlike Roger who topped the tour rankings. And Pete's last Slam title, at the 2002 USO, was over an old rival whom he had owned, and not over any of the upcoming Slam champions of a new generation; that, again, was unlike Federer's Wimbledon victory.
Federer's longevity has arguably been greater than anyone's since Laver and Rosewall: and I'm including Agassi in there because Andre achieved his late-career success partly because he took a few "walkabout" years in the 90s that saved his body. That is in contrast to Federer who has been playing nonstop, entering all the Slam events, for his entire career.
Tilden, Rosewall and Gonzalez did succeed into their late 30s. Pancho and Ken had strokes, and movement, about as efficient and easy as you can imagine. I get that you appreciate that; I do too. But I can't imagine judging Federer's longevity by that yardstick. There are too many differences.
For one, Pancho benefited from significant periods away from tennis. So did Tilden: he's a particularly bad example to compare against a modern champion because he was an extremely late bloomer. Before 1919 he succeeded very little, and served time in the military, all of which saved his body for his (genuinely great) efforts in later years.
Rosewall did not benefit from any such time away from the grind; and he matured very early, as you know. And he was still threatening to win majors in '74. The more I think about this longevity question, the more I see Rosewall standing out. I used to think of Tilden, Pancho and Ken as pretty much indistinguishable in terms of longevity, but I think now that there's real evidence to give Rosewall the top distinction in that category. He seems to have lasted mainly because of his wonderful style.
But before I get too far off topic -- I was talking about the differences between past eras and today. The game today is a runner's game, as everyone acknowledges. Again, it's not a question of today's game being better. Even those who are partial to past eras can agree that today's game is all about retrieving and grinding -- on hard courts, to boot. And in such a game there is a high premium on the legs. The very instant the legs start to go, in today's running game, a champion will begin falling behind. He can keep up to some degree if he has other options -- as Federer has. But I think any open-minded observation of today's game will make a fan sympathetic in judging how well these players are keeping up, as they age, against a field of retrievers who are among the fastest men that the sport has known. The game really is more than ever about court coverage and speed: and it has very often been said that the first thing to go, with age, is the legs.
2008 and 2009 were the first years that I remember people starting to speculate, or say, that Federer's court coverage was declining. It was still a subtle decline then: but as I said, in this runner's game, the very first drop in court coverage will have an outsize impact in results.
You could see in the 2009 Wimbledon final, against Roddick, that Federer was still serving as well as ever -- perhaps even better than ever -- but that his receiving game had clearly begun to diminish.
As for today, it's not even a question anymore, if you just watch him and then compare against the first Youtube highlights you can find from his peak years. He was not necessarily #1 of all time in sheer footspeed but he moved with an energy and dynamism that even I had forgotten.
Now whether there was some decline before 2009 is a more difficult question. Indisputably, his win/loss record began declining significantly in 2007. In that year he had far fewer victories than in '06, and twice as many losses. Whether that was caused by any slight loss in the legs, or just in the fact that nobody can keep up the pace he had set in previous years, I don't know. But I disagree with you that a great player cannot begin to decline at 27. Sampras, whom you have in the first tier in the GOAT debate, turned 27 in 1998 when his numbers took a clear downturn and his body started giving out on him, particularly toward the end of that year.
(I recall Federer having back problems that may have affected his serve, in early '09. Don't recall if that was his first such trouble.)
These are all general arguments and I still want to post something, on which I'd like your response, about playing level. I've already posted some of it above, showing Federer's high winner/error ratios at his peak. There are a wealth of stats, in modern tennis, that can illustrate a player's level in any given year or across a period of time.