I agree that that is the ideal end situation, but Unicode would have been dead on delivery if they had chosen that approach. Memory just was too expensive at the time to make a system that, in most of the computer-using world, wasted 75% in every text string. And no, just-in-time decompression wouldn't have worked either; CPU cycles also were too expensive at the time to do that.

Unicode also would have been too incompatible with existing code that copied 8-bit character strings around. See http://www.cl.cam.ac.uk/~mgk25/ucs/utf-8-history.txt for some rationale behind UTF-8.