I'm a signal donor and while I disagree with your point regarding UI (have you used in the past couple of years? It's went from feeling dated to feeling pretty modern), I agree with the rest.
Even worse, though, is that the EU offered them the opportunity to become relevant on a silver platter, by forcing WhatsApp to open up their app and be cross-platform with others who want to. Signal said no thanks.
I get it, WhatsApp stores metadata, and Signal doesn't like that. But they were fine with (way way worse) SMS for a while? The day Signal chose that path was the day Signal willingly chose to be irrelevant for the vast vast vast majority of people.
I love this app but the way the project is managed baffles me sometimes.