I have no reason to believe that the RT-AC5300 won't eventually get updated. Whether they will have AiMesh support implemented is another story, but I think they did mention it was being worked on, the three radio design causing additional issues they need to sort out.
The irony here is if you click "backbone" in the "seamless wifi experience" section at https://www.asus.com/AiMesh/ , it is clearly showing a tri-band router using the 2nd 5Ghz band as backhaul.