Saudi Arabia league will soon be as competitive as Bundesliga or Ligue 1 if they continue recruiting players that way. They started with Cristiano Ronaldo and every day you hear about new name coming to play there. Mendy, De Gea, Neves, Koulibaly, Firmino, Kante, Niguez, fofana...are names that joined or will join Saudi Arabia league!
Right now there's no chance that Saudi Pro League would be as competitive as any of the top leagues in Europe. Quite rightly they are alluring players from Europe, but most of them are either old and past their peak, or they aren't just good enough players, mind you that the Saudi Pro league is yet to allure a world class player that is in his prime.
Having said that, remember that what the Saudi's are trying to do right now is somewhat similar to what the Chinese Super league did a couple of years ago, but we all saw how it ended, i wouldn't rule it out to end the same way for the Saudi's.