People that only watch football on TV and think most clubs in main 5 leagues are total garbage have no clue how football works. You don't get out of the lower leagues without some kind of tactical prowess and talented players. Also these are professional players, who are in good shape and they all believe they deserve to be where they are. Even teams that had a shit run of games (Valencia under Neville, Internazionale under De Boer) - those weren't bad players there. Sure, most of them weren't great players, but cohesion in the squad is done by good transfers and a coach that has things under control. We've seen the same players after playing a lot better, because they were used better and they also wanted to prove themselves they know how to play football.
Because believe me, all of these players know how to play football, the biggest drop off comes in what they aren't good at. Dumfries can be great in defence, but indecisive at most times in attack with only the cutbacks being his good way of crossing. He might be infuriating to watch at times, but he isn't a shit player. Same goes for Nagatomo, dude wasn't the greatest talent out there, but at least he was a worker bee and coaches love players like that.
There are so many players that come from lower positioned clubs to clubs like Inter, Juve, Milan, Napoli,... And they become stars there.
......
Also, its not like even the title winning sides didn't struggle here and there. You know all of this, you've been watching football for over a decade. You've seen minnows having amazing years, maybe even winning a title, you've seen big teams struggle against the last team in the league even tho they just had a great run of games against rivals, you've seen great teams crumble under the pressure.
Atalanta haven't won anything in recent years, yet I've admired their persistence and development of players. Sassuolo is the same. Fiorentina too, I hate them for being a satellite for Juventus, but other than that I can't call them shit. Most of the lower clubs I'd call average and most of them we win every season, but they are still a certain level of a challenge. You need to put your best foot forward in every game. Lower teams don't just let you win.
So what is my point - these are players that have been drilled into good footballers since a young age. There is surprising amount of good, serviceable players in Serie B as well. Yeah, not all of them have the mentality to be successful in big teams under pressure of title hunting, but that doesn't mean you can't win titles with players brought from Dinamo Zagreb, Cagliari, Atalanta, Udinese - because that is exactly what we did not so long ago.