Of course, that assumption is correct. I agree that Real Madrid is very difficult to beat when they qualify for the final, in fact Real Madrid's final record is still very good since they last lost in the 1980-1981 season. Real Madrid lost to Liverpool at that time with a score of 1-0 thanks to a goal from Alan Kennedy. This is history, but since then Real Madrid have never failed to win the Champions League title every time they have qualified for the final. In total Real Madrid has won 9 titles in the last 9 finals in the Champions League, which is truly incredible regarding their record.
It is not assumption, it is the fact. It is true that Real Madrid always won in their last 9 UCL finals. Although there is no guarantee that Real Madrid will always win the next final matches, but they already prove it for years that they are very superior in the UCL finals. I think it is because Real Madrid knows how to build a strong mentality among the players. So, when they are in a crucial match like the final UCL, they still can play calmly and never lose focus on the match.
So I agree with you that, if a team wants to thwart Real Madrid's efforts in the Champions League, then they have to do it from the knockout stages including in the last 16, quarter-finals or semifinals. It's hard to expect Real Madrid's final record to be broken, but it's not impossible in the future.
Guardiola ever stated "
any team wants to win UCL, they must beat Real Madrid". It is clear that Real Madrid will be always an obstacle of each team that dreams for the UCL trophy. Even Guardiola admits it, he understands that Real Madrid is the most superior team in UCL. However, to beat Real Madrid, it shouldn't be in the final round. Yes, it can be in round 16, quarter final, or semi-final. But it is just in theory, I guess someday there will be a team that can beat Real Madrid in the UCL final.