Earlier quoted context omitted.
It's a desktop. Use an external disk. Much cheaper.
It kind of undermines the sleek form factor if you need to have a clunky NVMe enclosure dangling off the back though. Even with this tiny new design I bet they could fit a hatch on the bottom with space for a 2230 M.2 drive, but they don't want to because that would let you upgrade to 2TB of fast internal storage for $200 instead of $800.
I can't imagine anyone but Apple shareholders drooling at the taught of overpriced soldered memory would prefer a smaller Mac Mini case if ~0.5" more height would get you M.2 bays for storage.