Lots of people like having just one messaging app, for messaging that's basically SMS-like. Apple and Google have both chased that dream for their SMS apps, for a good reason (Apple with more success than Google—does Google still have another messaging service attached to their SMS app, or did they give up on that when the first attempt was a disaster about a decade ago?)

You don't! But that's exactly the problem - you don't need signal at all. All the important messages that you get are gonna come through another platform (SMS, whatsapp, weechat, etc [depending on where you are]). So, now that SMS support is gone, all signal installs are in addition to the apps that you actually need.

Most people will use the messaging platforms that they need to have installed to get through their days. It's nice, ofc, to have friends who are privacy enthusiasts - but Signal main goal has always been to go beyond that group.