if the key management systems don't interoperate, the services don't interoperate.
if the trust networks don't interoperate, the services don't interoperate
if i get your matrix messages, and stick them in a plaintext RSS feed you will find out, and i will lose your trust
storms and teacups.....
on the other hand, will this make meta up their game? that's a business decision, which I am not qualified to answer. but i think it might at least create an environment where some services may choose different business models, so that they can (up their game)
"to the extent that the level of security, including end-to-end encryption where applicable, that the gatekeeper provides to its own end users is preserved across the interoperable services"
by the way, many people have most the apps on their devices, so if those apps have open APIs, client side (secure) bridging is trivial (could put it in an enclave/trustzone if super worried about some apps being leaky) - could also use federation to build distribution trees for secure comms (with keygraphs).