a side note. make sure you checked out the list of supported aax plugs before you make the jump. there may be some that you want that you wont be able to use w/ zero latency until the developers release in aax format.
The HDX PCIe card tech is certainly not going anywhere, and the HD interfaces are surely gonna stay HD Interfaces with Digi-link Connectivity, for at least, the long haul. I have no clue what Avid is doing in their R&D labs, but I think it is safe to assume that Avid can and will qualify 3rd party [or build their own] Thunderbolt Expansion Chassis for HDX and HD Native PCIe Core Cards that will help interface their existing systems and achieve greater versatility for the bleeding edge of new computer technology.