I think you're right - tho let us also take pleasure in the moment.
He has just done what no other 'Blairite' has dare do, he's quit and stood up for his 'principles.' He's done what Michael Portillo never dared do v major, and regretted ever since. He's positioned himself as the person to take on that mantle.
If Brown hangs on somehow, he's just lost a few months extra salary (and the opprobrium of pushing through not exactly popular welfare benefit changes), but he'll be a leading candidate immediately after that.
If Brown goes now, he'll be the only likely Blairite candidate. And he wont win (it'll be Johnson) but he'll be the next home Secretary instead. Again, not a bad place to start from, even if/when Labour still lose.
e2a: I see David Milliband has said he wont be resigning, clear run for Purnell.