Of course Apple should not make a games machine, but a general purpose Mac Cube that can be upgraded would have generated excitement. Allow an Nvidia graphics card or make an Apple one.
The Mini and Studio are general purpose Mac cubes. The sticky part is "upgrading" and, yes, at a minimum Apple should support upgrading the storage (which
is removable and
technically upgradeable) and/or add extra M.2 slots to the Ultra version (which has the spare PCIe lanes to drive it).
RAM is tricky, because there
are speed and power advantages to having LPDDR RAM soldered in with the shortest traces possible (...the plug in LPCAMM system for LPDDR RAM seems to have sunk without trace). The real problem there has been Apple's mean base RAM specs & absurd BTO upgrade prices which deter people from just getting plenty of RAM on day one - but with RAM prices now going crazy, Apple's prices are looking less outrageous by the minute. We'll have to see whether Apple does the decent thing and gives up a bit of margin to absorb the price rises.
A Mac with a discrete GPU featuring NVIDIA or AMD's latest desktop chipset is going to be... about as good at running AMD/NVIDIA-optimised software as 101 other PCs with the same chipset, and probably deeply mediocre at running Apple Silicon-optimised code.
Apple need to encourage developers to optimise for Apple Silicon iGPUs so the resulting software performs well on the MacBooks which are the core of the Mac market.
Apple Silicon gives MacBooks, iPad Pros and Minis something special that differentiates them from the PC world - and also gives the Mac Studio a Unique Selling Point for applications where having 128GB+ of RAM directly shared by the CPU, GPU and NPU beats more RAM bottle-necked by the CPU. Building a "me too" desktop/workstation to go up against a wall of established competition
in a declining market niche would be pointless.
I suspect that the existing Mac Pro is just there as a solution for people with existing, PCIe-based workflows - I suspect that, for a lot of users who just need the PCIe for specialist audio/video cards, even the M2 Ultra (needed to provide the PCIe bandwidth) is overkill on CPU/GPU.