I think that's a huge one.Financially it’s a fantastic move for him but football wise a big question mark . Leaving the guidance of the best coach in the world to play for a club with a constant scatter gun transfer policy and a manager who has won a couple of trophies with PSG but failed outside the confines of the French league.
He may look around and see that even very good young players often end up with middling careers. What if he doesn't make it at the very top? Chelsea have given him a contract that should set him up for life, which takes that pressure away.
Now, as well as a fresh start (and there's definitely a bonus to being the shiny new thing - expensive players often get in ahead of "free" youth players), he can at least focus on making it as a top footballer, knowing he'll likely never have to worry about money again.