I agree with some of the factors that made PSG fail in the Champions League this season, but when you talk about titles then I think it's a bit difficult to achieve even though you have a lot of experience.
True. In history, only Real Madrid were able to protect their trophy in Champions League. Others failed even they were still very strong in a season they had to protect their trophy. Barcelona, Bayern Munich and many other strong clubs failed. Barcelona in gold time with Messi, Xavi, Iniesta failed too and it is good evidence to support your opinion.
Strong opponents, luck, team mentality, experience and strategy of the coach are some other reasons to judge a team's success in the Champions League. You might also be able to say why did Benfica qualify for the quarter-finals, why did Villareal qualify while Barcelona, Juventus, PSG, Inter, AC Milan and several other teams failed? You have to know what the reason is. There's a reason for everything, and I'm sure the few teams that made it to the quarter-finals now have the luck factor despite their best efforts.
In Champions League, you need both strength, good strategy and luck. Without luck support, it is impossible to win Champions Leauge. It is not like in national league that is long and lasts in many months. So if you make mistake, loss in a match or few ones, you can come back and even win trophy.
In Champions League, you don't have such opportunities in quarter, semi final or final. One or two matches decide result.