Without seeing the "guts" of the Karma it is difficult to say

I have seen the guts of a Karma. I'm not saying that it would have been impossible to make the battery user-replaceable, but it would have added significantly to the size, percentage-wise. Also, presumably, the cost.

IIRC, there is no "battery connector" in the Karma -- it's soldered to the board.

To make a battery flap, you'd need a few millimetres if you wanted to make it robust enough.

And then you've got to allow that the batteries aren't simple stock parts. I don't think that they were specifically moulded, but they could have been. There's not just a AA cell in there . Admittedly, you probably could source them from Farnell...

In the future, batteries will start to be moulded as part of the case -- then they really will be difficult for the user to replace.

And, as others have said, the battery management in the Karma is better than that in the iPod (at least the earlier ones anyway), so the battery ought to last a lot longer before it needs replacing, anyway.