Obviously salary is the main attraction for moving to Saudi Arabia. Messi rejected the offer mainly because of opposition from his family members. For the European players, it maybe quite difficult to get adjusted to the rules and cultural norms in Saudi Arabia since there are no pubs and bars. On top of that, playing in the Saudi Professional League may not give them the same exposure that they may get while playing in different UEFA leagues. But all of this will go in the background, when Saudi clubs are ready to pay 4x or 5x the current salary for these players.
No I don't believe it, Messi want to join Inter Miami because he get more interesting offer e.g. receiving share of Adidas and Apple, also we're know if FIFA is always favor to Messi since he won few controversial Ballon d'Or. Since he win the world cup and his performance is still looks outstanding, this give him a high chance to win the next Ballon d'Or. If he join Saudi Arabia team, FIFA will not favor on Messi and this could make Mbappe or Haaland will win.