I believe that there are some Mikrotik-specific features that still need to be supported under ath79 for the boards to be ported. As I understand it, the ar71xx code base will still be present on 19.07, though will be removed from master at some time in the near future.
It is my belief that the Mikrotik boards should be supportable under ath79. Someone with the boards in hand and the skills and desire to port/backport the necessary features will be required.