The hand-wringing evidenced by some about the difficulties of introducing a new behaviour seems pretty feeble when you consider that Google has successfully intruded both a new transport protocol (QUIC) and a new application layer protocol (HTTP/2 i.e. SPDY) in a few short years. Rolling browser updates are now the norm and enable these major changes.
Browsers are not the only thing that speak http. There are a myriad of libraries used by API's and all manor of automation that must also be updated to support a new protocol. A good example of this is SNI. All major browsers support SNI, but there are plenty of libraries that have yet to catch up to that very well aged protocol.
API endpoints tend not to be apex records, indeed are characterised by using a label of "api" or similar which doesn't automatically conflict with other services, certainly not in the common case. It'd remain a violation IMO to use a host name as a service selector but in practice they could continue using address records for many years of transition.