New EU law could require iMessage and WhatsApp to work with other, smaller platforms

I don’t get it. I don’t want to use Facebook, so I try to convince people to drop WhatsApp and move to Signal and other apps.

I fail more often then not and it’s infuriating that WhatsApp is the de-facto standard for communication here, but how does sending messages from Signal to WhatsApp solve any problem? Why would anyone want that?