That happens when public uses it, en masse, in the first place. And as it is today the "public" has no reason/incentive in switching to it and that naturally keeps the "non-technical public" away from it.
So who is this serving eventually? A very few, mostly "technical public", in complete contrast to what you have said.
I suspect Signal's goal is something entirely else. For them it's just two things:
1. Some sort of shallow ideology (started with the founder and the flame kept alive by the later leaders who often come across ideological groupies; the last bit is a bit strong but whatever)
2. Executing based on that for the sake of executing that because they can
What their intention is not: Signal ever becoming IM app of choice anywhere and possibly they might have their reasons for this. One of those reasons could be affordability as you have said it.
Note: in case this wasn't clear, the moment you attach a "phone number" you make it inherently unsafe for most, like 99.353959353%, of "general non-technical public" to use Signal and remain really unidentifiable. Once they are identified - in most countries (and now even in places like USA it seems) only thing needed after that is being "picked up" and rest will be just sung even without a device. I mean it's so absurd to even not think about it is that it's ridiculous. So no, it is DEFINITELY not "non-technical public can use".