I am not sure that MIL-C-675 AR is more durable than glass... It could be, but I've never verified this. I put a Jimmy Fu crystal into one of my AR runs, and I could see a difference between it and the sapphire crystals in the run, but there was a definite improvement over no AR even with the sapphire-optimized coating applied. I will gladly do a glass-only AR run any time there is demand for it.
FYI, I have now officially changed coating companies. The new company will be used for the upcoming double AR run in early December and for the forseeable future. Two very exciting developments are the following: full aperture coating (i.e. no lip marks), and a higher performance coating. Previously the coating I used produced an average reflectance of 1.25% across the visible spectrum, and less than 1% at the sweet spot (550 nm). The new coating is going to produce less than 0.5% reflectance across the entire visible spectrum which means the crystal disappearing act will be even more profound on double coated crystals. The only difference is going to be that instead of a subtle blue, the new coating is going to have an even more subtle (harder to see) violet tint. So it'll actually be more color neutral. The only time you really see the tint is under florescent lighting, and even then you almost have to be trying to see it - and if you take a trip into an AD, I think you'll find the same situation on gen watches. That is, you're not going to see the color of the AR very easily, but you're immediately going to see what looks like an invisible crystal due to lack of reflections! (Except of course on PAMs where they still only coat the interior)