Apple either needs to introduce an upgradeable headless Mac, or open up the damned OS already.
With respect, no they don't.
What they need to keep doing is what they're already doing, which is making rapid gains in marketshare in both desktops and laptops and developing what looks like it could be the most innovative mobile platform around.
Set in that context, I don't think they really care about the few enthusiasts who would like a headless minitower. After all, how many average users upgrade their machine during its lifespan, apart from maybe some RAM and a bigger (external, usually) HD? Virtually none, I'd wager, apart from hobbyists (who aren't really the bulk of the computer market these days). Most people just buy a machine, use it for a few years, then buy another.