Of course PSG will not let Messi go just like that, PSG will do whatever it takes to make Messi stay in Paris. But the final decision lies with Messi, he could return to Barcelona when his contract with PSG has expired.
Messi, who is one of Barcelona living legends, still loves the club that has made his name, especially now that Barcelona is being controlled by one of his closest friends. Xavi has asked Messi to help him rebuild Barcelona's strength, this request is very difficult to refuse because Messi and Xavi relationship is like family.
Actually I still liked him at PSG until he finally decided to leave at an older age and nearing retirement. The reason apart from that at this time is very difficult for other clubs to bring in Messi with a large salary as for Barcelona, it is still difficult, I think, because apart from having financial implications, this will also disrupt their current harmony.
They currently have Raphinha and Dembele in the same position as Messi and obviously when he comes back this position will be up for grabs and it's very difficult for sure for Raphinha and Dembele but I really regret this if it happened.