Joselu wasn't sent because of Mbappe, he was sent because of Endrick. Endrick is joining this season as well, meaning that he is going to be a bench player, they already have the next generation lined up at the bench right now, with Arda and Endrick that I know, but I am sure there are other very young players too.
We have seen Vini Jr. being there when Ronaldo was there too if I do not remember wrong, they overlapped for a season or so I think, meaning that Real gets young players for next generation to play among the veterans. I am sure that their current squad is great, but by the time Mbappe is 30, Endrick will be like 25 or 24 I think, so it is going to be great for them to keep doing this. But that meant, Endrick would get bench minutes, Mbappe will start, so Joselu was not needed.
Maybe, that could be the scenario, yes, as you said. however, previously the option to make Joselu permanent was quite widely reported, it was even reported that Joselu was exploring other options even though Real Madrid was willing to make him permanent from Espanyol, but considering that Real Madrid would welcome Mbappe and Endrick, Joselu apparently chose to join the club in the Qatar League. yeah Mbappe's presence plus Endrick's entry into the main squad adds to the queue on the bench, that's why I'm quite sure that Joselu thought about it before he chose an offer from another club which ended with him leaving. Jeselu is not a star player, but with Real Madrid he was the key to their squad's victory. moreover, in a crucial match. that is also Real Madrid's option, because Madrid still needs a pure striker when the main players have difficulty scoring goals. moreover, it is not certain that Endrick will be given many minutes to play. at the very least, he needs adaptation before achieving chemistry with his teammates.
However, theoretically, with the presence of Mbappe and Endrick plus several other players as team requirements, Real Madrid becomes a much more competitive team. however, we cannot yet speculate too much about how they will play match after match. but what is certain is that Real Madrid has players who will become long-term regeneration, they have dangerous young strikers as well as their midfielders. well, maybe next Madrid will focus on updating its defense sector.