Way too many people talk about Mbappe making the team better, and I agree that they are going to be better there is no doubt about that. But not enough people talk about the fact that what could go wrong as well. I mean think about it, what if Mbappe arrives and the team has issues? After all this team won the league and the UCL without him, so they may feel like he doesn't deserve the attention that he gets and maybe not even pass to him when he could score, and Mbappe could be unhappy with his teammates and try to score himself or just not get along with them.
When you think about these chemistry issues, if that happens then we are going to end up with a situation that will not be easy for them at all. I think the better result would be just ignoring all of this as much as we possibly can. I am not saying that this will happen, all I am saying is that this isn't unlikely and we could definitely see something like this as a potential as well.
If you consider that without Mbappe Real Madrid won the UCL trophy last season as well as they won the La Liga tournament then Mbappe might not be in the regular XI next season. If you are thinking like that then I would say your thinking is completely wrong because Real Madrid didn't wait that long to put Mbappe on the bench and a player like Mbappe definitely won't sit on the bench if he is fit to play.
I don't know if you noticed this last season but it is clear that they did well last season without a finisher but there is no guarantee that they will do well every season. Of course, a good quality finisher is needed in the team to make up for the lack of a finisher in Mbappe. There is no doubt that Mbappe is going to be an important member of Real Madrid's regular XI next season.
Since every position of Real Madrid is already strong, Mbappe will increase the strength of the team.