They haven't signed Cristiano because he's the best they've signed him because the most famous. If he didn't sign a new contract he would've played in a different country but SPL didn't want to lose him so he's signed a mega contract. Al Nassr had to keep Cristiano.
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.
Cristiano Ronaldo is the one who stands himself at a place where the world needs him where the world demands him not that he needs anyone anymore. I believe that the place where the Saudi professional league stands right now is completely impossible without Cristiano Ronaldo's presence. Because the following and the fame of the Saudi professional league is all due to Cristiano Ronaldo. I think this is the reason why Al Nassr wants Cristiano Ronaldo as part of their league. If Ronaldo leaves Al Nassr then the following of Al Nassr will decrease immediately. There are other famous and capable players in the saudi professional league but it is impossible to beat Ronaldo's calibre.