They might kill off MOBI as soon as they feel comfortable abandoning all K2/DX* owners. Probably only a few years away.
AZW3 is currently the bleeding edge as far as the K3/K4/KT know, as well as however many never-connected-to-WiFi Pw1/PW2/PW3/KV's there are. All seven of which Amazon are not going to cut off support for, going by their track record.
* -- I don't know if there are
any owners of an Original Kindle, though.