How does that work for WhatsApp (a Facebook app now)?

It doesn’t. WhatsApp is an app on your phone. That’s it. It can’t be anything else anywhere else, it’s how it’s been designed.

That’s exactly my point, I can’t justify deleting it.

Some people shift to Telegram / Signal if they can get their friends to install them.

I did this. Shifted the whole family to Telegram, and am hoping Signal catches up so we can switch to that.

I do have a few members of the family on Signal already though. They're just the ones that don't mind giving up stickers and the nice interface

Signal has had stickers for over a year now. https://signal.org/blog/make-privacy-stick/

About the time they added stickers, and message replies my friends group chats switched over.

The feature exists but the overall quality and variety just aren't the same. For whatever reason, people get comfortable with the familiar and even a cosmetic change creates a barrier.

I have no doubt signal will catch up though.