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.
I also disagree with the fact that Messi allegedly single-handedly created the success of a team like Barcelona. Barcelona was just as strong as a team and their famous tiki-taka just requires great teamwork and the same great performers that were available. Therefore, of course, it is foolish to say that all the recent successes of Barcelona are solely the merits of Messi. Let's not forget that the backbone of that Barcelona was also an important part of the Spanish national team that became world and European champions. In addition, let's not forget that Messi is not getting any younger, and therefore his factor as a player also weakens with time, so the Paris Saint-Germain management should definitely keep this in mind.