The problem with AV Foundation and the deprecation of QTKit is not so much playback.
DXV is not a QTKit codec. Resolume does not need QTKit to play back a DXV encoded file.
Resolume also doesn't need AVFoundation to play back a DXV encoded file. Really, it doesn't even need to have the DXV codec installed at all. It's our own codec, we know how to play it back so we play it back ourselves.
The one thing that was great about QTKit, is that it was used in other apps *to create* .mov files with 3rd party codecs. So you can export a DXV encoded .mov file straight from After Effects or Cinema4D or any app that supports QTKit.
With QTKit being deprecated, and AV Foundation not supporting 3rd party codecs, this means that eventually, you won't be able to export DXV encoded files directly from After Effects. Noticed how FCP and Motion already dropped support for 3rd party .mov exports a while ago? That was when they switched to AV Foundation. That's pretty much the only real downside to the whole thing.
As far as I understand it, this is what the HAPinAVFoundation framework is meant for. If a developer wants to *create* HAP encoded files in their app, they can use that framework to do so.
If you have all your assets in a certain format already, the whole point is moot really. DXV will always be best for Resolume, other codecs will be best for other apps.