Player are faster and fitter because they are better trained and conditioned, it has nothing to do with the modern game, it's a function of sports science and is true in every single sport. It's not tactical.
And Pogba does contributes defensively, he contributes as much as someone like De Bruyne or Silva who I suppose are seen as modern 8. And you are not going to tell me that De Bruyne primary role is to be hardworking that's nonsensical, not every team play or has to play like Liverpool. And in general your premise is wrong because there isn't one way to play football in 2019 and you create your own balance by taking into account the qualities and flaws of your own players whoever they are.
You are too caught in trends like the 352, inverted wingers, playing it from the back, gegenpress, Tiki Taka or having two attacking fullbacks. These are trends based on few succesful teams, they are only really relevant for these teams and their players. City, Liverpool, Real Madrid and Barcelona play differently with a different dynamic in midfield despite all mainly using a midfield three and that's because they have different types of players around that midfield, different attackers and different defenders.
If anything we need to stop watching other teams and focus on ourselves, create our balance which if successful will be one of these new trends, that people will call modern game.