Most of the remaining left-wingers are in safe seats (although not Cruddas, interestingly), so if there is a wipeout at the next election, the profile remaining will shift quite sharply leftwards. Not that that will be much help to anyone by that stage.

I think Milliband or anyone will find it difficult to convincingly present themselves as a sharp change from Blair / Brownism while continuing to serve those policies in the Cabinet. Having been fooled once by expecting Brown to break with Blairism once he got in, but that then not happening, I don't think the party will buy it again. Milliband's attempts to subtly position himself will only heighten the creepy impression that you identify.

Of course, if the economic gloom persists and Brown can appear to be acting decisively, he might start making up opinion-poll ground on the Tories and keep the job himself. It's an ill wind, etc...