The reason why people talk about the SPL is because Cristiano signed for Al Nassr. Other players followed him because Cristiano opened the door to make it cool to play in the SPL. Other players got rich because European team could not match SPL pay.
They have targets and ambitions by bringing in these expensive players, apart from improving the quality of the league they also want to attract the world's attention and so far it's working because almost all of us are talking about this league. The main highlight is that Saudi Arabia wants to host the next World Cup and I wonder if that target is achieved will they stop bringing expensive players to the league? Obviously no one wants to spend a lot of money without a target especially if the target has been achieved.