I'd love an expandable consumer Mac.
I think if there's one hurdle, it would be the cost in establishing a market for "aftermarket" products, since there currently isn't much of one.
For example, the major graphic card developers only make Mac-compatible hardware on Apple's request because aftermarket sales are too small to bother getting into. And this issue may be unavoidable unless and until Apple pulls out of their 5%... or at least until they become a viable gaming platform in the eyes of hardcore gamers.
It works in PC land since PC users are used to choices. Apple holds a pretty tight grip on their platform. They'd have to loosen up the reigns as well.