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.
If I look at Banzema's performance this season, it's very unlikely that Banzema will be substituted, I'm optimistic that mbappe will be paired with banzema, most likely Ancelotti will do a 4-3-3 formation, for sure Banzema will be the spearhead, mbappe will replace the position Vinicius, and Vinicius will be pulled back a bit, if Real Madrid manages to bring in mbappe next season, it is certain that the real Madrid team will be even more fierce next season