This is the boat I'm in. My version of Signal already updated and encouraged me to switch SMS out of the app, which I did. Now I'm sort of split between these two apps; my family is, for the moment, still using Signal for both, but I expect they'll soon enough be forced to use Android Messages, at which point we'll have little reason to continue using Signal.
Once my immediate family is out, I expect it'll be a domino effect with my extended family and friends -- those of us on Signal will have fewer and fewer reasons (ie, individuals in our graph) to use it. As much as I'd like this to not be the case, I think it will be. A smallish percentage of my contact list was on Signal, but every few months, another few people would join. I expect this trend will reverse.
Once my immediate family is out, I expect it'll be a domino effect with my extended family and friends -- those of us on Signal will have fewer and fewer reasons (ie, individuals in our graph) to use it. As much as I'd like this to not be the case, I think it will be. A smallish percentage of my contact list was on Signal, but every few months, another few people would join. I expect this trend will reverse.