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.