They've signed Cristiano because ppl who're following him on social medias will follow SPL. They needed Cristiano to give SPL exposure because they wanted to host the world cup. Al Nassr's got popular because they've got Cristiano. They've kept him in SPL because advertisers & sponsors want to have association with SPL.
Al Nassr gave him a mega contract because their popularity's going to drop if he leaves. They know Cristiano's got commercial appeal.
I would say that if Cristiano is no longer with Al Nassr, then even the popularity of the Saudi Professional League is going to get destroyed. As of now, SPL is being watched in every nook and corner of the world. And Ronaldo was the one who started this trend. There are other superstars in Saudi Professional League at the moment. Karim Benzema, Aleksandar Mitrović, Pierre-Emerick Aubameyang and Abderrazak Hamdallah are playing in this league, if I need to name a few. But none of them can match even 10% of the popularity and fanbase that Ronaldo has.
The Saudi league without Ronaldo in it will certainly make the SPL less popular, so with Ronaldo in the SPL, of course this is a special attraction. but it feels unimaginable if Ronaldo leaves the SPL, of course the Saudi League will be like before where its popularity is not like it is today. but fortunately Al Nassr did something good by extending Ronaldo's contract which finally made the SPL still popular with many people.
it is true that of course Ronaldo's fans are certainly more than the star players you mentioned and of course it is natural that Ronaldo is very worthy of being maintained by the SPL. but in this case of course the SPL is very great because he did something good and of course now the SPL is very profitable as well as Cristiano Ronaldo who of course gets a fantastic salary and other benefits. yes I hope Cristiano Ronaldo can bring Al Nassr to win the trophy in the coming season.