Of course, it would be better for Real Madrid if they can get Mbappé, Harry Kane or Victor Osimhen. But the problem is Victor Osimhen is not leaving napoli. Tottenham is trying hard not to sell Harry Kane and that’s why they are giving him a huge price tag. And we all love the drama around Mbappé. I think Joselu will be above average for Real Madrid. But I don’t think being above-average is going to be enough for Real Madrid.
Joselu needs to adapt before he becomes a core player and that's why it takes one other striker as the main player for Real Madrid, so Joselu will learn to adapt to the team and will try to improve his performance slowly. Mbappé, Harry Kane or Victor Osimhen has not yet gotten a picture of the future of the three in the next season, because it is too complicated to the negotiation process of the three players with their respective clubs.
If in the next season the three players are not successfully brought to the Bernabeu, the available options are only Joselu and want or not Ancelotti must maximize the available players, if it is like that and there is no other way for Ancelotti. The transfer market is currently a lot of drama that occurs and the class of Real Madrid which is usually quite easy to bring in players to be dizzy because of the difficulty of reaching an agreement.
There is a variety of reasons why they need a second (or first) striker besides Joselu. First of all they need quality backup who doesn't need too long to adapt or even better, who is readily available to jump in if needed and perform well. A club with the goals that Real Madrid has can't afford to be without a striker at any phase in the season. Bayern had so much trouble because they thought they could improvise and fix the striker issue with some of the players who have good striking qualities but are actually not real strikers. It didn't work out as they were often left without solutions in the game.
Joselu would be the backup if you ask me but Real Madrid has to wait and see what will be possible on the transfer market. The situation is changing on a daily basis. But the second reason is that a team needs competition for any position. A striker who knows that there is nobody else to take his position may be less inclined to play his best. Competition is always good.
And lastly there is never a guarantee that a player can cope with the Real Madrid pressure unless it is Mbappe or someone like Ronaldo. They don't need any time to adapt, but Joselu isn't like that or maybe he proves me wrong.