Thats just really a problem for guys who buy a Mac Pro for themselves. And come on the OSX side there where never that many expansion cards to begin with.
And if we are talking enterprise, HP, Dell, Fujitsu, all of them could seal their machines to no upgrades internally and no enterprise would really cry. Those machines are bought, and a few years later are completely replaced. No one is going to the IT guy and asking yeah can I get some more RAM sticks? You get to hear wait for next years replacement. And more disks in those workstations? There you dont save anything on the drive of the workstation its all in the network.
And if we are talking enterprise, HP, Dell, Fujitsu, all of them could seal their machines to no upgrades internally and no enterprise would really cry. Those machines are bought, and a few years later are completely replaced. No one is going to the IT guy and asking yeah can I get some more RAM sticks? You get to hear wait for next years replacement. And more disks in those workstations? There you dont save anything on the drive of the workstation its all in the network.