Whatsapp is such a big cultural force that many of my non technical friends and family members refuse to use anything else. No mail for full res photos is one thing, but you can't make them use anything else. Everything lives in Whatsapp. Todos, notes, reminders, sharing stuff (airdrop is the only exception if everyone involved has iOS). It is less because of whatsapp features (Nobody uses any features like payments)…
Uuh, where do you live? I haven't met a single person (technical or non technical) with such an attachment to a single app in my life.
All of my friends in brazil, for instance, are entirely attached to whatsapp.
I agree with the other reply. Where do you live? I'm surprised that you haven't encountered any. Every city in the US that I visited and every city that I visited internationally has people that attach themselves to one of either whatsapp, telegram, imessage, etc.
(I'm kind of excluding wechat here because wechat seems to be forced, but there are plenty of people who have this attachment to wechat as well because of its status as an omni app)