Real Madrid have always been a good team in the Champions League, and I believe they will continue doing that as long as they will continue to be with Ancelotti. The Champions League competition is in their body already; they have been doing very well in the competition. We will see how they perform next season again, although I know the arrival of Mbappe will surely bring more positive changes from next season, so I think we should be expecting another Champions League trophy from next season.
Currently, Real Madrid is in a good shape. Ancelotti builds Real Madrid squad with great players. So no doubt that Real Madrid probably succeed again in the next season. However, there is no guarantee that Real Madrid will be the winner in UCL again. Other teams in UCL are also getting stronger, they know well that they will never win UCL trophy if they don't improve their squads. So, even if Real Madrid already signs Mbappe, they can't underestimate other teams.
It is good that Mbappe already joins Real Madrid. Now, Ancelotti has the expected striker for Real Madrid. Mbappe is one of the strongest strikers in the world, he should replace the role of Benzema as the new machine goal in Real Madrid team. I just hope that Mbappe won't play individually, he must have a good team work in playing. He can't play very individually like in PSG anymore!