Benzema can surely play for more years however I doubt he would be able to play like this in those years also. Because I don't remember Benzema playing this much awesome in the previous years too. Of course I don't mean that he was bad then. But this is like a career-high season for him. And I don't believe he can have a season like this anymore from now on. And when Mbappe comes, Benzema is very likely to be in the subs unless Real Madrid change their formation. Of course Mbappe will need to earn his place in the starting eleven but I don't think that it would take a very long time.
Well, why immediately send Benzema to the reserve? I wouldn't be too surprised if Benzema and Mbappe are on the football field at the same time at least next season. If, of course, everything is confirmed and finally Mbappe's transfer to Real Madrid is officially announced. Another question is that I am really not sure that Benzema will be able to play at such a high level in the coming years as this season. Still, betting on him for the next few years is risky enough, given the age of the player. But while Benzema is in such amazing playing and physical shape, it will be a real crime to keep such a player in reserve and give preference to other players.
He is 34 years old and at least a couple of years he will be near the peak of his abilities. Modern medicine and recovery techniques are progressing very quickly and if 10 years ago 34 years old was the age of retirement, now football players easily play at that age. Benzema's speed will begin to decline first of all with age, but he never made the main bet on it when scoring goals.