I'd argue that $5/month is quite high. Not because Signal isn't useful, but because the actual incurred cost per user is much, much lower. Of course, not everyone is going to pay. But if Signal were to ask users to pay, say $10 per year , i'm sure there'd be a lot more payers.
Remember, that Whatsapp in its early days used to ask for 1USD/user/year.
I take that as a lesson.