They don't talk the same protocol but that doesn't prevent them using compatible forms of message encryption.
If the basic concepts of "E2E key exchange" and "pass this encrypted message" exist at all points, I see no fundamental problems with having E2E encryption across different networks. I can see potential for lots of the normal practical small problems but it could fundamentally work.
I agree that it's technically feasible but that's not going to help me right now. It would require all services use the same encryption (or at least understand a common, compatible one) and I don't see that happening, ever.
If the basic concepts of "E2E key exchange" and "pass this encrypted message" exist at all points, I see no fundamental problems with having E2E encryption across different networks. I can see potential for lots of the normal practical small problems but it could fundamentally work.