I quite agree with what you said that the quality of the Saudi Arabian league is not good enough and they will only be a place for old popular players who do not get a place in Europe. Their advantage lies only in funding where they are willing to pay old popular players to play in their league to get exposure, but I don't think this will last long since people are only focused on these players not on the quality of their league. Still, they have to develop their natural players so that this league is not only filled with old popular players, but also good talents who have European-class football skills. It's not easy, but they should have the money to back it all up.
Slow death said that it is not yet at the level of Europe. It will never be at the level of European top leagues because they are not competing with the European top leagues. This is the same for every other sport where one or a few nations have the upper hand and the best domestic leagues. If another league from far abroad tries to reach that level, they would need the intense competition that these top leagues have domestically. You can't copy that or imitate it by bringing in some old former top players hoping that they would magically raise the level of players in domestic leagues.
This idea won't work out for Saudi Arabia, but for them it was mostly about the attention, not so much that they were hoping that Ronaldo would bring the whole league to a higher level as that was unrealistic anyway.