Read Dan Bernstein's retrospective on qmail, where he essentially disavows "least privilege" controls on his programs (which were the heart of his security model). Modern operating systems all offer some degree of privilege revocation and code-level access control; none of them get used, because users needs are too complex.

If a dent is really going to be made in this problem, it's going to happen in Flash Player (or its more recent analogs, like Google NaCL).