Jon Cruddas may indeed be "best placed to re-engage with Labour's grassroots", but only because Labour's grassroots are, in their hearts, leftwing and old Labour. New Labour did not emerge from the grassroots. It owes nothing to the class-based strongholds and the union-patronage that Jon Cruddas sees as the way forward.
No, New Labour's very success was brought about by ignoring the voices from the grassroots: by reaching out into brand new middle-class territory and by cooling down the unions (fairness without favours).
When Cruddas harks back to membership figures of 400,000, he forgets that this surge in new members, was simply an expression of support for a new kind of centrist one-nation politics. They did not want to become part of the old grassroots, attending pointless tedious meetings. They had no desire to evangelise on cold doorsteps; they had modern lives to lead.
The structure and practices of the party were so old, and set so rigidly in the past, that their only appeal was to minority political cliques. Consequently the new intake drifted away. And the party is back were it started. Labour has successfully reformed its politics. Now it needs to reform its party.