What I found interesting in this discussion was a power path through a TB port in addition to the built in power supply. That way, you could add an external USB PD battery and use it as a UPS.
...or you could get a
UPS (or a generic power bank with a mains A/C inverter) and use it as, well, a UPS - with the advantage that it would also keep key components like displays, external storage, network switches running in an outage.
It's not that there's
zero situations in which a battery-driven Mini would solve a problem - it's just that it's not a high-priority problem - risk x consequences - that the majority of people want solving on its own. I'm sure I've accidentally disconnected the power cord from a computer (not a Mac Mini) at least
once in the last, oh, 20 years or so, and maybe taken advantage of being able to keep working on a laptop during a power cut on a similar number of occasions (with modern workflows, though, that's not much help if the network is down, so it's time for a cup of tea*)...
An important production system needs a full UPS in the same way that it needs a fire extinguisher (low risk x major damage), but a UPS is just not really necessary for a personal computer, unless you have a problem with power cuts (which, again, demands a proper UPS that can keep your infrastructure going). It's not as if yanking the power on a computer is guaranteed instant death anyway - it's just something risky to not make a habit of. If your cable management is bad, you risk yanking network, disk drives etc. which also risk disruption, if you don't save & back up regularly you're at far greater risk from crashes and fat-finger errors than power disconnections.
Not that I'm opposed in principle to USB-C power
as an alternative but I really don't see Apple implementing it
without also removing the internal PSU, which would be bad. I don't want to "waste" a high-bandwidth I/O and display port just for power.
(* A way will be found. I'm British and pre-millennial - in the event of a crisis, flame will be kindled and tea will be made... with
boiling water... somehow.)