Player training generally can't keep up with level gains. Typically you can gain about 50 xp per season (without using tokens), but you need 75xp to maintain the same level of stars. If you only buy players just shy of 6 stars then the next season you'd average 5 2/3 stars, then 5 1/3 stars then barely 5.
At lower levels it's fairly easy to just replace most of your squad. At higher levels, those just-shy-of-6-star players become harder to find, necessitating a change in strategy. I still turn over about 1/4 to 1/3 of my squad every season.