To be fair, Real Madrid is still the winner, and on top of that winner team they also added Mbappe, so I understand why people may think that they are the best and they will win.
Plus, if any other team ended up being great then that is one thing, but Real Madrid plays specifically for this tournament, their entire team effort goes to winning UCL, they won it so much, more than double the next team, so they will do very well and probably win. I am not saying it is 100% guaranteed for them to win it, but 60% is definitely reasonable. There is a good chance that they could, but it still means 40% they will lose, so it is not like something that is guaranteed. I feel like they should just let this team be as great as possible and stop doubting their talent.
If you refer to what you said, then Mbappe's existence does not mean anything for Real Madrid. Before there was Mbappe, Madrid was 100% victorious. The proof is that Real Madrid won the UCL and LaLiga this season. However, if Mbappe is present, the chance of winning is 60%, of course this will be the beginning of failure for Kylian Mbappe. With Mbappe's presence, the level of prediction of victory should be higher than before.
Kylian Mbappe's career is a problem if he is not able to defend the UCL and LaLiga titles in the coming season. This is a tough challenge for Kylian Mbappe, because Real Madrid can win without Mbappe.