To say 100% that Real Madrid will win the Champions League next season, I'm not that sure either. However, here we can only see that their percentage of winning the Champions League title again is very high compared to other teams. I think this is a very normal thing, with the status of defending champions and the addition of their strength, one of which is by successfully bringing in Mbappe in the season.
If you calculate the percentage, last season Manchester City was the team with the highest percentage, but in fact they also failed because Real Madrid managed to beat them. Real Madrid overtook them in percentage terms and managed to become champions.
This could happen next season, even though Real Madrid may be the team holding the highest percentage, but that doesn't mean they can automatically become champions.
It is obvious that we can't say 100%, while I do think that they are the best team in the world right now, adding Mbappe made sure of that, because they were already at the near top levels, you would say either Madrid, or maybe City, but they were still pretty top level team anyway.
However, we are seeing how things are changing, and I think it's quite obvious that we are seeing some changes here and there now, which means that it is not going to be that simple. Mbappe addition put them at the top for me, but even the best team could lose, so I am not going to say 100% guaranteed title win. I would say it is high chance though, I would give them 50% chance to win it and I do not think that it will be quite different than what we expect.