I've got a few things to mention, in defence of Ellie and the rest of the staff at Feral and the folks at Aspyr:
1. The Nvidia 600 & 700 GPUs are now pretty old and the 680M & 780M apart are quite slow. I could imagine that even the 680M & 780M are
marginal on performance for these very high-end games (Mafia 3, Hitman, DXMD) even assuming no software problems. Most of these games require a full-fat Nvidia 660 GTX which is now slow in the Windows world, but faster than a lot of Mac GPUs.
2. Understandably focus for driver development tends to be on the shipping and forthcoming GPUs - GPUs that last shipped three years ago aren't so Nvidia Kepler support is being maintained rather than fundamentally improved. As newer games demand fundamentally faster GPUs it will become less and less relevant to games developers.
3. Nvidia are still plugging away on the WebDriver but I suspect they aren't able to sell as many full-fat GPUs to Mac users as they did in the past as there is no traditional Mac Pro. Being an unofficial add-on can make it harder to support - I've never officially supported it for UE4 for example - because developers typically aren't allowed to build Hackintoshes and effort tends to focus on the typical users rather than outliers. If eGPUs are successful then this situation would change if lots of Mac users start running Nvidia GPUs again. Everyone who wants Nvidia to stick around should go out and buy an eGPU box and a Pascal GPU!
4. Apple haven't shipped any Nvidia parts even though Maxwell and Pascal have a >=2x performance per watt advantage over equivalent AMD parts. I'd be surprised if AMD were selling at such a steep discount that the performance per dollar was better for AMD parts. OTOH look at consoles and how that has played out for AMD vs. Nvidia. Ultimately Apple's reasoning is immaterial, they aren't shipping Nvidia parts and don't seem to plan on doing so in the short-term, so Nvidia driver support is dependent on Nvidia's goodwill which is not inexhaustible.