The very best of yesteryear could adapt, thanks to having the sheer talent to do so. But most wouldn't be able to handle things like the generally increased discipline, faster matches, Better fitness, different equipment, more TV pressure so a lot of the old defenders wouldn't exist. Like, you can't stomp on a guy, apologise and get away with it like some used. Nowadays, you'd get a red right away, or TV footage would cause you to get a ban.
Anyone can adapt. This is basic human nature. And look the line up of a team like Real, they use players like Gutti, Helguera in the defense, they are average or bellow. So, any average and bellow can adapt (Deny human capacity of adaptation is a joke).
Jern is right and no one should add anything else to this topic.
Football has changed and is still changing so its natural if that happends they mght find it different but to suggest they would struggle is stupid. There are some ppl in here who think football now and the players of now are better then the legends, those legends from the past were better then most players in their time, just like todays excellent players better then the average ones now.
It's not just a matter of being "better" or not, there are UNDENIABLE differences in the level of professionalism today. Fitness of players is much better nowadays, diets are better, players don't drink and smoke like they used to. The game IS faster, just watching the games proves that. And that isn't taking into account different social circumstances and pressures.
Isn't the whole point of the thread "what would happen if someone plucked the Peles and Beckenbauers from the past and put them right into today's football?". To simply say "they would adapt" is naive at best. It's a different question to say "what would happen if they were born in recent times", because obviously they would be different players and different people.
The truth is the question is a generalization, today Pele could die with 10 years old accident.
But all the changes you liste : Fitness is a matter of players being grow with that, Diets also, Players drinking and smoke the same (Yeah, sure. Pele did not drink and smoke at all), The Game is faster only happens because the superior fitness. So, it would be a matter of adaptation. Nothing new and It is really simple, not naive.