I highly doubt that he will be able to play at his highest for two more years. but I think we will see a similar kind of Benzema in the next year also. and with Mbappe and Vini, the attack of Real Madrid is going to be one of the best in the world. But I honestly believe that Real Madrid really desperately needs to get a good Defender.
Otherwise, I think they will be conceding a lot of goals. and that is certainly not ideal. If they have to get rid of some players for that I think it will be worth it.
Mbappe might be a good addition to partner with Benzema and Vinicius and I think maybe mbappe will be the right replacement if in the future Benzema is no longer productive, but as you said, if madrid's front line is no longer in need of much change because indeed it's good enough, I agree that now ancelotti really needs additional players in the defensive sector, especially central defenders, after Ramos left for PSG, so far there has been no player who has been able to guard the madrid defense as well as ramos, even though madrid have had Alaba but I think he hasn't can find his best performance, maybe if Madrid can bring in Rudiger next season and partner him with Alaba it is certain that madrid defense can be quite solid.
Mbappe Is going to be one of the best addition to the squad in my opinion. and he will have age and time on his side so he should not have too many problems playing for Real Madrid for a long time. I think after Benzema he will be the main striker of Real Madrid. and he is flexible in terms of position so I think that is also not going to be a problem for him. and having someone so flexible in the attacking is going to help the team a lot.
And about the defensive side, I also think that Rudiger can be a great option for Real Madrid.