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.....
Or, instead of all that upgrades for MP 2013, for the same money, you buy a house.