I think An Nassr is currently benefiting from Ronaldo's branding rather than his game. Ronaldo coming to An Nasr has brought money in other forms to the club. I think Ronaldo is still very profitable for the club, even though in terms of playing he has greatly decreased.
This is what he has now been taking into consideration for quite some time, it is not unknown to Ronaldo that age is after him and he won't be able to play forever. But many of the critics have to admit that he is doing much better than almost anyone else at his age. Ibrahimovic was an exception and there are very few other players who were able to play well, but there has never been a player I think who was still able to perform on that level not only in the Saudi Professional League, but in the national team. He is still scoring against some of the best players in the world and for as long as he is able to do that, he will be part of the national team and also able to play the national team competitions like the upcoming WC.
That's why he's still trying to keep fit, avoid things that would help him look weaker and then stay healthy, it's not really much about Al Nassr at this point but being healthy enough to participating in the next world cup, that's a trophy he's yet to achieve in his carrier so he'll do all his best to help his team go far in that competition and try to win the competition if possible.
Well Al Nassr still needs him not only for the branding but for his contributions on the training ground and pitch, during match days, he still scores a good number of goals which makes him important to the club, that's why they increased his contract.