Maybe that's why they recruited Messi. As we saw that with even a bad squad like Barcelona's Messi was able to do wonders. Barcelona can now feel that how important Messi was for them. He single-handedly carried them to success. Maybe that's why the PSG board thought that Messi will be able to help them get the CL trophy. So far this season PSG do look like a contender for the title, let's see what the future holds for them.
Messi was a great player for Barcelona, prolly the best to ever come out of camp nou, but I'd not say that he singlehandedly carried Barcelona to glory, I know he was their most important and influential player, But Barcelona is a great club and they had a lot of quality players, Messi made them absolutely complete, but he wasn't everything, he also needed other players to function as well, what I mean is, Barca and Messi had a seamless relationship. Just take a look at how things are panning out in PSG for Messi, he isn't at his best cause he does not seem to have the sort of connection he had at Barcelona. Having said that, I still don't think PSG are somewhat ready to win the CL, they have the individual talents, but as a team, they are not the best and it could still take them a while before they are CL champions.