> Some folks seem to think the future is with so-called 3rd generation cameras that are mirrorless,
Yes. The flappy mirror in a SLR was all about putting a viewfinder in the same line of view as the film focal plane. Don't need to do that with a digital camera, cuts way down on parts that can misbehave or break, also cuts down on weight. They need to improve two things to make mirrorless work for me: 1) viewfinder responsiveness, and 2) focus responsiveness. Both are about processing, and I don't think Moore's Law has flatlined in cameras, yet.
> and won't necessarily work with either FX or DX lenses.
Depends on how important that is to the camera manufacturer. For Nikon, compatibility has been a priority, so I think they'll continue to support as many of the Nikkor mount generations in their mirrorless line as is practical. But, a lens built for the focal plane offset of the camera will mechanically work better, usually, no stupid pet tricks to translate.
All that said, I'd have to get either a lot better at photography, or a lot younger, or both, to switch out of DX...