Except that Signal is free and nothing prevents anyone from having both installed at once, so you being stubborn can get your contacts to install the free app that takes two seconds to install.

Then everyone ends up on Signal because anyone can install Signal but the stubborn people refuse to install WhatsApp, at which point "everyone you actually need to communicate to is on Signal, WhatsApp is pretty much useless."

But in order to get there, you (the collective you, the median pedant) have to be more stubborn than the people who want to use WhatsApp, instead of the other way around.

Most casual contacts aren't going to install Signal just for you, no matter how stubborn you are. They'll just shrug and go their way.

I've been there, actually running Signal as my primary IM for several years. The number of people I "converted" who stuck around was, in the long run, zero.