The Mac Pro that you keep on pining over is NOT that expandable -- in fact the Mac Pro 2013 is much more expandable. I have a Mac Pro 2008, two graphics cards (5770).... which take up all but the lowest speed PCIe slot (a SATA card in there). All 4 hard drive bays full..... no more expansion possible. With the Mac Pro 2013 it has the two graphics cards in there and up to 36 Thunderbolt devices.... so add a Thunderbolt 4 drive bay (and I have 35 left). Or add a 24 bay one and still have 35 left.....
There's faulty logic here, and thats assuming that in a standard tower, you wouldn't Also have expandable options.
But if you built a tower Mac again, you'd have the additional drive bays inside, additional PCI-e expansion for standard devices. and then, on top of that, just like the 2013, THEN You can continue to expand outwards with expandable devices.
so even with this logic, no, the 2013 Mac Pro is by far, infinately not more expandable than an equally specced hardware in a standard ATX (like) tower form factor. It's far, FAR more limited as this case, you can't easily swap out the existing internals, like drives and CPU's.