Joselu is no longer young and even this season he didn't get a main place so when Mbappe comes later it's clear he will be left out, but I think Joselu is still needed by the team, to be a cover on the front line.
For the front line, I think Vinicius Jr and Mbappe can still be a partnership or play on the wing for one of them and indeed if you look at the quality I think anyone between these two players can play on the wing or in the middle and what is certain is Vinicius Jr and Mbappe's attacking instincts strong enough that I don't doubt them when playing up front.
Joselu is a midfielder just like Bellingham but you know that Anceloti like to multitask his midfieldera anytime the forward aren't making enough contributions to break the defensive system of their opponent, that was how Joselu was used to penetrate Bayern Munich towards the end of their semi final when they were leading the match. I think that Anceloti will definitely knows how to make use of him by next season especially now that he has said he is going to retire in Real Madrid.
Benzema before the Saudi Arabia use to be a winger and was converted to striker, I think Anceloti might just have to convert one of them that has both left and right foot and then use the 3 of them as forward. It's going to be a fantastic mix and collaboration as this help strengthen Mbape relationship with the club and last longer than everyone would have envision.