Other World Computing (OWC) actually sells a device called a miniStack which is essentially that. An external enclosure the same form factor and design as the (last-generation) Mini. It's still uses an older USB 3 connection though, no daisy-chaining.

That’s been a thing from a lot of companies for past Minis (e.g. https://www.newertech.com/products/ministack_classic.php ); I see no reason why it won’t happen again.

Contemporary SSD storage only takes up as much space as a pack of gum, just like sticks of RAM. Even SSDs with terabytes of capacity! Any desktop computer, even a small one, has all the physical space needed to have upgradeable storage.

Yes, you could fit one m.2 SSD slot into a Mac Mini.

But anyone who has a serious professional need for large storage (e.g. video editors), probably doesn't just want 2x-4x the internal storage, with the same bus-factor-of-1 fault-tolerance. No, they probably need either a Thunderbolt RAID array (that they can put into RAID5/6/10), or a SAN. And neither of those fit in a Mac Mini.

(Though, a Thunderbolt RAID array of m.2 SSDs might fit in a Mac Mini-sized enclosure. Does anyone sell that?)

Those already exist but it's kind of missing the point. An NVMe SSD is the size of a stick of gum, I'd much rather plug one of those in than have to stack a box the size of the Mini on top of my Mini and connect it with TB3. In the end you've got something more expensive, probably slower, and less portable than the obvious solution.

You could put a 3.5” drive in a Mac Mini sized box. And with Tunderbolt you could have it double as a hub or dock

Something that allows maybe 4 or more M.2 SSDs and 2 x TB3 controllers...