For a SFF? Are you trying to run a plex server or something off of it?
Just needing a lot of storage in my system while having to move the computer every now and then using a motorcycle - which is why I require it to be SFF to fit in a backpack
At the moment I have 1To in my main computer and a server for storage - I'm really happy with this configuration. I don't think SFF are made for storage.
But it's my point of view.
Given that I run my current build in an even smaller case than the C4 will be (namely, Dans SFX-A4), I disagree with you.
M.2 are the norms now are they not?
At least when I built my current PC, when the Ryzen 2700X got freshly released, most Mainboards only had 1 M.2 slot - Mine got two, but given the selection of MITX Boards I went through to finally see a suited one released tells me - relying solely on M.2 might be a stupid idea. I want the case to be small too, but if you wanna go the "m.2 only" way, why not get an even smaller case also rocking MXM graphics - those exist and are even smaller, too. I Hope you get my point - I feel like 2.5inch drives should still fit in - not a fan of the 3.5inch idea that came up a few pages back, because, after all, it's SFF - but going full m.2 only is a bit too strict imo.
The Way I see this case, is as the logical expansion of the A4, giving space to fit better cooling solutions in.