Here, in India if you mention SMS you get weird looks even from people who crossed their youth using SMS. Communication is WhatsApp here.
My physiotherapist, my doctor's nurse (who handles my appointment) they all function on WhatsApp, and no, they are even going to try Signal. Grocers, delivery people — it goes on. My bank and credit card relationship managers communicate with me on WhatsApp. They are never "migrating to" Signal.
I am the only one among my contacts who will be moving away from WhatsApp — everyone else (among my people who are using Signal sparsely or actively as of now) have WhatsApp installed and will keep it.
Most of my recent Signal convert contacts have already started replying on Signal after hours or days, some have uninstalled. Most of those people will soon uninstall it is what I expect. Unless Signal brings social features that makes these people experience FOMO of broadcasting their lives on one more network :)
Penetration of Signal in real world is way too different from what it "feels" here on HN.