The only SLR's I know of with penta-mirror VFs are cheap models, and the best way to improve VF performance would be to upgrade to a better camera, not to use heavy, expensive f/2.8 or faster lenses with such low-end camera bodies.
I agree that a dominant reason for lenses faster than f/4 is VF brightness but my guess is that current fast lenses handle such needs, so that new ones are not needed enough to justify the R&D expense.
As Michael's wish-list for the "next EOS-1 digital body" suggests, what is needed in lenses for the digital era is sharper, not faster, and if anything, it is a bit easier to design an optically excellent lens by staying away from large apertures. Many sources suggest that f/4 is about the limit beyond which optical aberrations become an important limitation on image quality.