Why? Cross platform encryption is solved in a myriad of ways. Only thing missing is consensus and key verification. I think apps live Threema show how it can be done. Lacking a central instance to bless keys one could verify in person by scanning some code.
It won't, not exactly. The law doesn't require that you be able to message between, say, WhatsApp and iMessage. It only requires that those services allow connections from outside their walled gardens by publishing APIs. Those APIs will spell out what's expected of the client in terms of encryption schemas.