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).
The Mac Pro supporting two SATA drives but no M.2 is pretty sad... but really, a Mac Pro can have dirt cheap PCIe M.2 cards.
A related issue is: if a Mac's flash storage entirely fails, the Mac is bricked, and there is no way of booting it off another drive.
I would prefer Macs to use M.2 boot drive; but other than that, I don't see any real problem with the mini and Studio not having internal storage expansion. There are so many external solutions that work fine, and there's basically no performance hit between internal M.2 and external TB4/USB4 40Gbps drives.