It’s not really about fitness per se in my opinion mate.
More about knowledge / education. Players today are fitter because we have far greater understanding of sports science / nutrition etc.
Available today the vast majority of great players would transition no problem.
End of the day you need to be able to play. As the King himself said - “.....if you cannae play football a bowl of pasta’s no gonnae change that”.......
I've had this conversation a few times over the years, and there's a couple of points that need consideration imo.
People in general were smaller, there was very little obesity around, a lot of lads looked skinny but were strong as f**k, so when people speak to me about "older" players getting bullied off the ball etc I tend to laugh, because a lot of these players were hard as nails and strong as a horse. They just didn't look it because they weren't bulked up, they didn't have the kind of "model muscle" as I call it, but the core was just as strong.
The other point is that these guys got absolutely hammered out on the pitch, I mean the tackles were absolutely brutal and I often wondered how they carried on, but they knew how to ride a tackle, their balance was superb and the could take the knocks.
Of course a lot of them now have problems with knees, ankles, necks, hips etc but a lot of todays players are going to have problems as well, because they are carrying weight which is not natural for them, in added muscle.
And then there was the ball; not one of the featherlight things they have now, the ball had real weight, and to get bend, curl, dip on it was a proper art, and was a basic of the game.
I'm sure a few of the older lads can tell horror stories of getting hit in the face by a wet "casey" or trying to kick the f**king thing when it weighed a ton, and that's before we even get to the pitches, some of which resembled a newly ploughed field.
All that, and those guys still played at a very high tempo, showing a lot of skill, pace, stamina, and everything else you expect from a top player.
There's a lot to be said about the progression of sports science, and all it entails, but at the end of the day, as in the above quote from KK, all that matters is "can they play", and f**king hell those lads could play.