Is a loss of trust in USD necessary for the success and mainstream adoption of Bitcoin?
There is not black and white answer here I guess, but here is my logic.

Let's say that trust is a proxy of security and stability and satisfaction is a proxy of liquidity, usability etc.
If we look at the extremes: if people had 100% trust in the USD, then they are less likely to look elsewhere for alternatives (holding everything else constant). Therefore less trust in the USD is more likely to increase BTC adoption and hence value.
However an improved usability, liquidity and other functionalities may be good enough reasons to increase BTC adoption as well.
Therefore, we can conclude that a loss of trust in not necessary, but it will help.