Real Madrid's performance will not decline next season either; they will gain more strength because of the presence of Mbappe, although even before Mbappe came in, they were doing very well. Furthermore, I think the Champions League will be very tough this season. Arsenal, Manchester City, and other teams will want to take down Real Madrid, which won’t be a difficult thing for them. But it will be too early to predict how teams will perform next season, so let’s watch and see.
But after all it is the Champions League and many of those games are always fiercely on the edge. Just see the games against Manchester City and it ended in a penalty shootout. It could have well ended otherwise. Dortmund was the stronger team in the first half in the final as well, but they just didn't have this little bit of luck.
But in general you are right that Real Madrid will be rather stronger in the coming season. Although a player like Kross is now gone and his value for the team was beyond doubt very high. Mbappe will add value of course, but I wonder how they will be playing and whether they play with three strikers or whether someone like Rodrygo ends up sitting on the bench.
And the transfer window is still open. It is possible that they will try to add even more quality.