Opinionated is good. It means everyone gets the same predictable experience and it's easy to set up. On the opposite end of the spectrum you'd have something like IRC, which nobody uses because it's so hard to get started.
Open clients/protocols/ecosystems create individual freedom and choices at the edges. One size does not fit all, even if you market it as an "experience".
My mother, a 70 year old retired special education teacher, got me and my extended family (21 people) on Signal. I didn't tell her about it, I didn't encourage her to use it, I didn't step her through the process. She doesn't know how to make a folder.
Signal hasn't beaten WhatsApp (which still uses Signal protocol) due to network effects, but it is a great solution for everyday users.
One such mechanism I often lament is that closed systems attract investment dollars seeking to capture a market then eventually enshittify, and that investment can be spent on marketing/polish/etc to drive adoption. I don't think this is the dynamic around Signal though, per se.
I can envision a world in which Signal still did everything they did to promote themselves, while also not binding themselves to mobile phones, Google Play, phone numbers as identity, secure enclaves, etc - where they were just as successful at gaining "normie" adoption, while also having a free foundation. This is where my criticism is coming from.
The last I heard there was a "desktop client" but it was actually going through the mobile client. This could be out of date, although I haven't seen anything directly refuting it and I haven't bothered to dig.
To the original point, the proprietaryesque policies rooted in the authoritarian desire for centralized have severely hampered my interest/adoption, and I still don't see how policies disrupting the libre ecosystem in any way encourage adoption in the non-libre ecosystem.