The smaller sensors have exactly the same issue. Take MFT as example as it is easy to do the numbers. Lets say a 20mm f1.4 lens (like the Sigma) vignets wide open a lot on FF. For large part due to the micro lens structure.
To get equally shallow DOF and similar FOV, you need a 10mm f0.7 lens for MFT. The angle of light to the corners will be comparable, and vignetting will be comparable.
Seeing the relative success of the Sony A7 range, in the mirrorless part of the market it is interesting to enter FF. If one does, only a mirrorless mount will sell. A DSLR without mirror will not be an attractive deal.
So, if they decide to enter the mirrorless FF segment, it will be with a sensible flange distance, as according to Canon management, mirrorless is about smaller size and lower weight. They then would need some shorter focal length more compact primes to make the mirrorless FF make sense, and the rest can be covered by an adapter + EOS lenses.
Time will tell what the FF part of mirrorless segment will do in marketshare.
The problem with mirrorless lenses really is with non-retrofocus lenses, especially wide-angle ones. The angle of incidence of light rays just gets too high, this si why I suggested a reasonable, not too short flange distance.
Personally, I'd like to get rid of the mirrorbox - EVFs have come of age, they are plenty detailed, and they even allow one to view things as they are outside, light level wise, or choose to see it as the image will be recorded.
As to aperture equivalence: I don't really care about that. One chooses a system for a reason. For extreme shallow DoF one choses a FF camera, or Medium or Large format. Besides, I don;t know whn you last shot a fairly tight portrait, but if you want more than a few eye lashes in focus with a lens normally used for portraiture, on FF you really need at least F/4 to achieve that, so an F/2 MFT lens will suffice, no problem.
If I really want very large apertures on MFT, I use my metabones adapters with my Canon glass, no problem, but in that case I specifically choose to do so.
I love FF shooting (own a 5D II after all), but I don't always want to carry it around when I am on the move, which is most of the week these days. And the IQ of MFT is very good indeed. In the end, in order to make a photograph, you need to carry a camera - and in my case I always do and want to do so, but not always a FF with the relatively heavy lenses I own.
Time will indeed tell what mirrorless FF will do in the market. The way it is now there are relatively few users, however. If Canon would come out with a 6D variant that is mirroless, I think uptake would become a lot better.
Kind regards, Wim