I imagine one could “paint” the back “wall” of each ring segment in a fresnel lens black, to significantly reduce the annoying light scatter, at the likely cost of thin, but visible black crescents… Question is what mass-production-viable process there might be to do this, whilst leaving the lens fronts untouched.
I believe Sony filed a patent application for a similar, easy to apply, but probably nowhere near as good, solution, where they would print black to the top rims of the ridges…
I think any “simple” optics would preferrably need to be concave on the user side, so that it wraps the eyeball - otherwise we get a fair bit of that other fresnel effect, where you transition from see-through, to total reflection and strong polarisation, with increasingly oblique angles of attack.
For this reason, I think one of the pluses the optics in the Index has, is that going dual element lets them make each lens plano-convex, eliminating the “hybrid lens” convex bulge on the user side. (EDIT: Ever seen the Index logo? A circle next to two semi-circles – an eyeball, next to a stack of two plano-convex lenses. :9)
Somnium says they are experimenting with an alternative lens train, that eliminates chromatic abberrations optically, but expensively…
I still imagine with sufficiently good eyetracking, dynamic software distortion compensation could do the trick.
Argueably, the 12k is the 8k evolved, and I suppose they could offer it in a variety of configurations, down to a simple 2x4k variant, without any of the fancy stuff onboard - just what it takes to drive the panels, and track the HMD…
They just got sidetracked with the Crystal, whether as a stopgap solution, to keep themselves going through 12k delays; Due to creative distraction; And/or bucking to persistent demand from this very forum. 
I have to mention that 12k marketing renders show a sandwich of two lenses per eye, like the Index, where the inner one is still fresnel all the way to the center, there right behind the conventional portion of outer one…
( I happen to have here a simple… contraption, where I for a bit of a lark duct-taped two pairs of cheap, spherical, plano-convex, 73mm glass lenses to a pair of huuuge cheap display panels, and frankly my vision in it does not strike me as all that bad – quite optically clear across the view, and not extremely distorted – an estimated 180-200-ish FOV, more stereo overlap than I’ve experienced in any real headset… Kind of wish I had the chops to write a driver for it, so that I could see what it would look like with a proper VR view… :P)