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.
I think Barcelona can do well without Messi so far, why Xavi not keeping his trust for current squad? bringing back Messi might make Barcelona players lazy and depend on Messi and give him hope of victory. Xavi must be able to convince himself as a coach as well as his players, because even without Messi they can win the title. In my opinion, let Messi choose a team other than Barcelona, however, i think he is already prone to injury and is getting older of course sometime age is not a indicator he will become bad players, but Barcelona already found their playing rhythm without Messi.