A Mac pro is the same size as a mid tower pc. My Toshiba laptop is the same size as a macbook. I have multiple hard drives which I swap frequently, and the laptop is no bigger for it. It has a removable battery as well. Those are the only two things I'm asking for in an upgradeable PMP.
I'd like it to come assembled and everything, I'd just like the choice to upgrade. My 30gb iPod is getting a bit small for me, everything still works perfectly though, I just wish I could drop in an 80gb drive for say $150 rather than $250 for a new iPod. Frankly I don't need a 30 and 80. SSDs aren't actually any smaller than HDDs. There just aren't any moving parts to screw up when I take it mountain biking, ATVing, or drop it while I'm working out. So rather than buying a nano for $250 I buy an SSD for $150. Again, frankly I don't need a nano and a classic.
Also you fail on the cars analogy, although spectacularly and only somewhat. They, uh, are component based, you can replace an engine. You don't have to buy a new car if you want to go faster or your engine blows up. Granted, most people can't do this at home, but I think that has more to do with lifting 1000 lbs of metal out of an awkward space and the complexity of the mechanics of a car. I think I could handle the ounce or two of a harddrive. Also think I could handle slipping a drive into a socket. Some people may be afraid of going inside the guts of a PMP but I have a feeling the average MC user wouldn't. We're the people that wanted more control than what we can get out of XP or Vista MC. We know our tech decently.