More exactly, this 24MP stacked sensor would have made more sense in A mount because they already have decent long telephoto primes for A mount. It would also balance between ergonomically with large telephoto lenses. You would have gotten 20fps shooting with the mechanical shutter and all the advantages that confers. The a9 seems a bit of a waste of an interesting sensor.
1) Nothing is stopping you from putting your A-mount glass on an A9. You can adapt it.
2) A-mount would not attract Canon and Nikon folks like E-mount does, as again, you can adapt your EF glass or FX glass on it.
3) Sony has never delivered more than a... 12 fps mechanical shutter, is that right? There is no 20 fps mechanical shutter ready to go.
4) Eventually a 20 fps setup like this could hit the A-mount someday (with an electronic shutter), but why they'd lead with that instead of E-mount is beyond me. Again: Trying A-mount is a full blown conversion, where E-mount is far less painful to try out.