I've been following Dan's products across several forums for a while and I'm completely on board with what Dan says above. Compromise is an accepted part of SFF builds, but for maximum compromise and minimal size the Dan A4 already exists. The original C4 idea might have looked sleeker/better (the latter is purely subjective of course) than what's being considered now, but it had a lot of the same limitations as the A4 (to the point it wasn't interesting to me at all). I can understand that must be disappointing to someone who really digs the original design, but as a niche in a niche product, that's a bad deal - and if Dan can expand the compatibility (and by proxy, the possible audience) so much by the suggested minor size increase, it would be madness to ignore this just on principle because "SFF is about compromise" or the hyperbolic "just buy an Ncase M1 instead".
Also, straight by numbers the amount of 2- or even 2.5-slot cards sounds like a lot, but if you check out the actual products, most of the well designed SKUs, even for mainstream cards, have gotten larger because it makes the cards cooler, quieter and faster. At the same time, many of the smaller designs are cheaply made and hot/loud. For a case that's not soundproofed while probably sitting next to you on your desk, that's pretty bad, even if we accept that you can only really have two out of "small, powerful, quiet". Getting massive improvements in terms of "powerful" and/or "quiet" for a minor hit to "small" is entirely worth it, to me. Because - again - the A4 already exists if I wanted to go smaller.