This relies on an implicit idea that a system being closed is required for encouraging adoption, but without actually stating a concrete mechanism for that.

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.