Neymar has already officially moved to Al-Hilal. I won’t be surprised if PSG’s Arab friends are behind this transfer - firstly, PSG got rid of a too expensive football player, and secondly, they not only got rid of him, but even received money for the transfer and thus improved their business in terms of FFP. As for Neymar, it is certainly hard to see the career of the once most promising player decline at the age of 31. I wouldn't be surprised if he completely finished at 33.
Neymar knows that he is difficult to play in a competitive league anymore because he often gets injured. Although he is still 31 years old, his career may end soon if he often gets injured. So, when he has a chance to play in SPL with a big salary, he immediately agrees to move there. He may think that it is hard to shine again with the injured problem. And by playing in SPL, he may play for a few years and collect good amount of money before retiring. Moreover, it becomes interesting to compete with Ronaldo and other top players in a new league like SPL. So, we can understand if Neymar decides to leave PSG and choose Al Hilal.