It's interesting to see the strong interest in AiMesh (or mesh in general) here. I don't believe it's the utility of mesh itself that attracts people but rather the fun to form a network by adding nodes and see them in action. Without AiMesh really shall not have driven people crazy..
When ORBI was hit on this forum, I made a suggestion in a post that a better play of mesh shall allow existing and old routers/APs to join as nodes. I don't think Asus read my post nor my suggestion was of any nobleness. Apparently though some middle managers in Asus and I thought alike in terms of having fun.
Back to the TM/AiMesh saga, seems like AiMesh was not intended for TM models to begin with. Looks like some programmers' oversight that this feature was accidentally made possible by flashing AC68U firmware on TM models. Just my guess. So this shall not be read as ASUS is obliged to support AiMesh on TM models.
Some people can't let go that's the case is a little bit beyond my comprehension. From a business point of view, the TM models are both done sales for T-Mobile and Asus. The contract itself is very likely not including additions of new features (especially a "major" one like AiMesh).
I wonder if there is deep resellers' interest on after market at play here.
Asus perhaps could use this opportunity to further boost the hype of AiMesh in a creative way. For example, charge an extra fee to cover additional licences and provision new features such as AiMesh. My guess is that TM users in this category is a small minority. Hence, the overhead in administrative cost perhaps won't justify it.