If we're talking legacy only, Mbappe would waste another year of his prime if he goes to Saudi Arabia. He'll be 26 at the end of next season. That's not old, but that's not young either for someone who has started to play at the highest level at age 16 and whose main asset is his insane speed. He may very well start to lose his speed at 30/31 (like Henry or Ronaldo) and won't be nowhere near the same player without it.
If we're talking money, well he's already the highest paid footballer (maybe athlete?) on the planet and has been making saudi level money at PSG. Nobody in football/sports history has made as much money as him by the age of 25. He's well ahead of Messi and Ronaldo in that regard. So if someone can afford to turn down these crazy amounts of money, that's him.