Rise of T20 definitely can't be held to blame for all of it. Add in the amount of useless white ball series' we play at the expenses of Test matches.
Whittling down test series' against NZ and Sri Lanka away down to two tests, seemingly because they're less commercially attractive.
The complete lack of tour matches we play these days. We played six tour matches on the Ashes tour in 2001. How many last year? None.
The reduced match experience the top guys and the ones in the wings get all adds up. It's an absolute myth that they play more cricket than ever. Go see how many FC and List A matches players back in the 70s/80s/90s would rack up and compare it to now, yet we've got batsmen now being rested when they're available for Shield matches.