No idea why any player with real ambition would want to join PSG and play in that little lwague.
I'll give you some reasons:
1. weather is nice in Paris (something his family would push for)
2. he'll find it easier to score/assist
3. he'll be in a team, which he wanted to join for quite a while (MUFC was his plan B)
4. ADM will get first team football - pretty much guaranteed
5. if he stays at MUFC, there is a real risk that he may not even make the bench and would hardly play.
6. if he can't play in the first team, it could affect his chances to play for Argentina
7. if he doesn't play regularly, it may affect his form.
Apart from money and the fact that he is already here, there is no other reason why ADM should logically stay.