The EU are doing exactly that, but they’re doing it the sensible way. Instead of pointing to a particular standard (Signal, RCS, whatever) and saying you must use this, they’re forcing Apple, WhatsApp, etc to publish open APIs that allow others to hook into their services. This allows platforms that are distinctive to develop but prevents vendor lock-in. Honestly, I’ll be all over WhatsApp and iMessage once I can use an open-source client to hook into them.