My guess would be that the main thing that happened is that it wouldn't be ergonomic anymore with 27" inch monitor sizes and such. You would have to look upwards and that could be unhealthy for your neck.
Anyway, I remember seeing a bunch of desktop-style cases from Silverstone and looked up the URL:
Looking at those now, I'm thinking these aren't truly "desktop" in the sense that you would put them under your monitor. They look more like intended for use next to people's HIFI stuff in the living room and such, or for mounting in a server rack.
The other thing I remembered is this one here, that's truly "desktop":
Does anyone even make traditional horizontally-oriented cases which go under the monitor anymore?
yes, all of the OEMs make tiny form factor PCs. HP, Lenovo, dell, etc. They scale from pathetic atom powered rd gateway thin clients up to i7s n stuff. even their 1u FF towers can be oriented any way you like.
R.I.P. Zawarudo, may you OC angels' wings in heaven.
If something appears too good to be true, it probably is.
Best R0ach Quote of all time : TLDR: Haswell might be the last legit gaming platform unless mice get their own non-USB interface on some newer architecture.