In this case one of two parties must join a new platform. Either they join a privacy respecting one or they can call/text. If they can’t respect that boundary further communication will not be required.
The network effect is a powerful force. If you want to talk to loved ones (or more and more, businesses) you are forced to use this platform, which hands inordinate amounts of power to the likes of Meta or other proprietary platforms that only ever have their own interests at heart.
I refuse to bind myself to proprietary platforms that I have no way to leave without insurmountable switching costs. Behaving this way is not without personal cost: It is more difficult to talk to me for others which annoys them or discourages them from trying. It also makes me look like a pedantic ass on platforms like these (although my phrasing there probably doesn't help I admit).
There are plenty of privacy and freedom respecting platforms that allow interoperability and I think it is better to use those rather than adding my bit to the network effect that makes WhatsApp the de facto standard, strengthening it further.
Because the rest of the world is not stuck to iMessage/SMS? We all adopted a messenger app back around 2010-12, that continues to work across all smartphones, computers and some feature phones, that has actual E2EE for one-to-one chats, even if the metadata now goes to Facebook instead of Apple.