Perhaps the Studio update is delayed so they can get Thunderbolt 5 in it, figuring the Mini doesn't need that? If they don't wish to update the Studio each year, perhaps waiting a bit longer to get the powerful new Thunderbolt standard in makes sense to them. It'll be interesting if an M4 Pro Mini is on par in performance with an M2 Ultra Studio for a few months.
I wish they'd make the Studio and Mini true desktop computers (the Mac Pro is extremely expensive and niche). I consider them 'headless' (display-less) and keyboard-less notebooks. Here's my reasoning from the longer term past:
Desktop Computer advantages over Notebooks:
1.) Larger, more powerful (and hotter, higher power consumption), higher performance processors.
2.) More RAM slots for DIY memory upgrades.
3.) More expansion (e.g.: PCI) slots for internal non-integrated graphics cards, HDDs, SSDs, etc...
None of those advantages apply to the Mini or Studio. The System-on-a-Chip approach prevents after-the-fact RAM expansion, and apparently non-integrated graphics have been written off (perhaps unless one goes the expensive and less efficient external GPU route? Not sure where that stands now). The performance benefit of desktop processors has largely disappeared. Larger systems may stay cooler and not throttle as much, but I doubt that impacts most users most of the time.
There's no sane (from a customer perspective) reason a Studio couldn't have a couple of spare SSD-compatible drive bays. Even one would be a game changer.
Perhaps shrinking the Mac Mini helps get rid of that extra space inside that begs the question of what consumers would like to see in there?
Question: if you had a choice between a Mac Mini the size of an Apple T.V. albeit a bit taller (let's even say it's got the same number of ports, though unlikely), or a Mac Mini in the current size with the addition of an internal drive bay (and you can make this DIY 3rd party internal SSD your boot drive easily), which would you pick?
I'm aware some people avoid Apple's price-gouging SSD upgrade pricing by getting an SSD and putting it in an external TB 3 enclosure (wonder why that's hundreds cheaper?). It often involves a performance hit, one more piece of clutter on the desktop, reliability issues and concerns about how hot some of them get (judging from the thread discussing these things) and would complicate management and backup (over having everything on one SSD).