Interoperability would help with then potentially making them switch.
But there’s also people who will refuse change and trust me I got many.
With how many rely on WhatsApp and here in Cyprus Viber you would be amazed. Interoperability helps so that in the event they insist on refusing to switch, at least you get to keep using signal and they get to keep using WhatsApp.
I kept advertising my profile for signal.
Only until Chat Control did I get a few but only 2 out of the tens (idk how much I have exactly in family, around 50 more or less) so yeah.