It is not surprising that in the end the Saudi league started to be more ruthless in invading the top European leagues and they did not hesitate to spend funds to get the players they wanted. The Saudi League has everything that European players want, especially large salaries and also the guarantee of abundant wealth if they want to sign a contract with a Saudi League team.
The league is growing but it has yet had everything European players want, it is not true to say so. Some players will want to join it especially old players but some other players will want to stay in Europe especially young players or players who are at best years in their career.
I agree that with very deep budget, top clubs in Saudi Professional League, can spend a lot of money to buy many star players from Europe. Their biggest challenge is to convince star players to join them as with very big transfer offer, clubs in Europe will nearly not reject offers from Saudi Professional League clubs.
It's just that this is not comparable to a more brilliant career in Europe by winning other prestigious titles and I think it comes back to each individual's view. They could come here for one or two seasons just to make a lot of money and if they still get the full attention of the teams in Europe then they will still be able to keep coming back later. The addition of slots for foreign players also proves that the Saudi league still has full control to control the market for all players throughout the world.
It is easy decision for old players to join this league and get a lot of money just within 1 to 3 seasons. It is more than what they earned in previous seasons of their career in some cases. With young players, they can do it as well but they have to risk their career by doing this. However, there is example like a Belgium player Witsel who moved to Chinese Super League then returned to Europe and played well for Dortmund.