This is interesting and I love how you put this. A lot was expected of Dybala and recently when Allegri or which coach was it spoke about Dybala, he said he likes comparing himself with him yet he hasn't lived up to it.
Bale fell off the wagon too and same with Hazard with his injury problems. A lot of these players lacked the determination that we can see in Ronaldo and Messi this is why they have stayed relevant for this long and others fell off. It's hard to see who would replace these players.