He couldn't stay at Barca, there's a Spanish law that prohibits paying people less than 50% than previous contract unless they've agreed to it in their previous contract. That's why he couldn't even had stayed if we offered to pay for free.
It's a bit of a myth that Messi hasn't tried for PSG, the issue is that their defense and midfield is awful.
Yeah he hasn't reached the heights of Barca, but that front 3 individually isn't PSG's problem.