Do you think Barcelona would still be better than PSG if PSG had Messi?(obviously Barcelona don't have Messi in this hypothetical)
Peak Barca would beat PSG even if they had Messi. They would be able to control possession and thus reduce the time Messi is on the ball. More than that, they would use pressing to stop PSG from giving the ball to Messi. Messi would still have the quality to make PSG win matches against Barca, but he wouldn't have the opportunity to do so except rarely.
People talk about Barcelona as if they are a collection of stars. When Barca football works, it's an example of great tactics, smart movement, aggressive defending and highly combinative football.