Exactly right. In fact, I will be doing this too. Right now, I use Signal as my default messaging app, and messages go via Signal to people who've registered their phone number with Signal without me having to do anything to effect that. When Signal stops working as an SMS client, I'm not going to look up which of my contacts wants to use Signal and which can only use SMS - I'm just going to use my default SMS app for everything except the couple of group chats which are on Signal already. Messages which would have been opportunistically encrypted are now going to go out plaintext, because the friction to find out who uses Signal and who doesn't is too high.
This already happens with iOS, because the Signal app doesn't work as a default SMS client. People who use Signal and Android reliably send me messages over Signal, whereas my contacts who use Signal and iOS mostly send messages over SMS unless they're directly replying to something I've sent them over Signal.
And even if I wanted to do the work to send everything over Signal to those contacts, how am I supposed to remember exactly which of my kid's friend's parents use Signal and which don't? Fuck it, the juice isn't worth the squeeze, everything is going over SMS.
I'll just have to hope that Google figures out a way integrate a seamless, opportunistic end-to-end encryption protocol into the default Android messaging client, since Signal is deliberately dropping the ball here.