80% is too low for Real Madrid's chances of winning in my opinion, why not give 90% more?
It is a personal prediction, it is no problem to give 70%, 80%, 90%, or even 100%.
However, we don't know what will happen in the match. The prediction has nothing to do with the real chance of winning the match. It will depend on the teams to play in the match. The team which is not ready for the match, they will have a smaller chance to win.
I know it is very unfair to underestimate Dortmund in the Champions League final, but since the opponent is Real Madrid then Dortmund's low chances seem reasonable. Dortmund have beaten many of the favorites so far in the Champions League including Atletico Madrid and PSG, but they have to face super favorites in the final which means they will have to work twice as much to hope for victory.
People view the chance to win the UCL final based on the stats of Real Madrid in the previous finals of UCL. People have their own reasons, I think it is okay if they favor Real Madrid. Moreover, Real Madrid has a better squad, it is normal if people assume Real Madrid has a high chance to win the final match of UCL. But it is true that it doesn't guarantee for the win. It will depend on how Real Madrid can play with their best in the final.
Everyone believes that only Real Madrid has a chance of winning, but it does not rule out the possibility that Real Madrid could still lose.
True. Whatever the stats of Real Madrid performance in the previous UCL finals, the chance to get a lose will be always there. So, it is surely not impossible if Real Madrid can be defeated by Dortmund even if Dortmund basically has a lower squad quality.