Unfortunately, ATX PSUs and 3.5" HDDs are the (literally) biggest blocks to downsizing your PC. People will hang onto those things for years...
Yes, but supporting those components doesn't preclude having a small enclosure by any stretch - at least when you're talking about cases in the neighborhood of ~10L. You just have to be willing to accept that you'll be able to do some of the things a case can support, but not
all of the things
all of the time.
What frustrates me is that I see many people with hulking towers for computers, who resist a move to smaller form factors,
even when it would result in a better experience for them. Just as many people tend to buy PSUs rated for wattages they'll never reach, I've found that (for whatever reason) the attitudes of most system builders tend to be that of "do everything you can today to avoid limiting expansion down the line". Overbuild today, "just in case". When in reality, they're spending a lot of money, and accepting a lot of compromises, for an option they ultimately never capitalize on down the line.
It's very easy for most to see the pitfalls of buying "not enough" - not enough performance, not enough power, not enough space - but less so to see the (equally detrimental) cost of buying
too much. And, when it comes right down to it, all SFF is really about is finding that middle path, that configuration that meets our needs and wants as precisely as possible. The
right amount of performance, the
exact quantity of storage and graphics performance we need, and the enclosure that provides
just enough room to house it all.