I didn't intend a comparison between Gmail and Hangouts, just to say a whole lot of people already had the required account.
You definitely had a rougher experience with it than I did, but my main point is Google launched it, didn't seriously iterate on it, and gave up its strongest distribution channel at the first sign of pressure from carriers. Since they keep launching messaging products, I must conclude they want to be in that space and it was foolish of them to squander their best opportunity.
Hangouts UX sucked big time. I remember lots of frustrating sessions with my parents about why video calls weren't going through, or how can some random family member join our family thread when they don't have a Gmail account etc.