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 should have already considered leaving the club after he saw the new players coming into the club and also should have envisioned that he’ll not be a regular player in the striking role again. He didn’t get enough playing time even in the concluded season, so he should know he won’t get now that more young and talented players have been brought in by the club. He should have gone back to his former club and continue there instead of staying here. I see him as a third choice striker not even a second choice if he stays back. Meanwhile, one will also think that since he’s no more a young player, he won’t get to be appreciated that much when he goes back to his former team. He did well to qualify Real Madrid to the finals, so they won’t want to dispose him just like that.