Presumably, people agree that a smaller desk footprint is a net good.
No - at least not significantly.
Even a Mac Mini vs. something the size of a honking great Mac Pro or mini-tower PC is only an "improvement" if you don't need/want the internal expansion space - otherwise you end up with multiple boxes for storage, PCIe, hubs etc. and a rats-nest of wires taking up more desk space than the mini-tower.
...and once you've got a decent system down to the size of the original Mac Mini, going ever smaller brings diminishing returns and often backfires.
The typical location for a Mac Mini/Studio is the dead space under the overhang of your display. It isn't taking up useful workspace, other stuff can be stacked on top of/under it (unless somehow you need to pick it up to turn it on, but nobody would be daft enough to design it like that /s) - trimming an inch off the footprint brings very little advantage - and the original Mini was already perfect for VESA mounting behind a display in which case it would have zero footprint.
...and making it smaller has other costs. Enough has been said about the power button - which simply wouldn't be an issue if they's kept the old form factor. The M4 Pro Mini has "lost" a TB4/USB-C port c.f. the M2 Pro Mini - probably due to space constraints - so more people are going to rely on a hub/dock
which takes up more desk space. Both Minis have also had their two USB-only ports moved from the back to the front - whereas
extra front ports would be useful, these aren't extra - they're a replacement for the two rear USB-A ports (plus, remember, the M4 Pro has lost a port overall) and people who previously used those ports may now have to have devices permanently connected to the front - trailing cables over previously free desk area (or, again - get a hub, more desk area). Having cables sprouting from two sides of the machine isn't going to make VESA/under-desk/rack/security/etc. mounting any neater, either.
Now, the base M4 has arguably
gained connectivity looking backwards - but looking at the M4 iMac we know it
could probably have had 4 TB4 ports - obviously the M4 now has 4 rather than 2 TB controllers - so its not clear why the M4 Mini only has 3.
Maybe the M4's extra TB controllers have come at the expense of the "spare" PCIe and USB lines used for 10G ethernet and extra USB ports on the old Mini, but that's not in evidence, whereas it is clear that the new design doesn't have space for a 4th TB port on the back (and missing out a TB retimer chip probably saves space on the mainboard).
We'll have to wait and see about the thermals - I doubt the M4 will be a problem, but the M4 Pro... well, Apple have messed up thermals for the sake of "smaller is better" in the past (2016 MBP, 2013 Mac Pro) but hopefully Apple Silicon will be more forgiving.
Another driver is net-zero carbon emissions. Lighter, smaller, or both, reduces its carbon footprint. A good estimate of carbon footprint is just from a product's weight.
If I want to reduce my carbon footprint I'll stop buying prepared salad in plastic tubs. A new Mac every 4 years or so barely registers - and all that valuable aluminium in Macs helps making them
worth recycling and helps ensure the other nasties get properly disposed of. If Apple wanted to make themselves "greener" then they should have offered a M4 logic board upgrade for existing Macs. As it is, "smaller is better" is a major driver and justification for non-repairability and planned obsolescence.
Heck, the people who want to easily access the power button can just place the Mac mini on its side.
Seriously?
Then it
wouldn't tuck in under the display and
would take desk space that is currently unobstructed (and the stand would need a fairly large footprint to keep it upright against cables plugged in to the 'top rear'). Plus, they make the top smooth shiny silver and the underside black & covered with slots and labels for a reason... the alternative to "form over function " shouldn't have to be "to heck with form"...