Tony Blair was not an MP in 1979. David Cameron was not an MP in 1997. The chances that the next Labour PM is a current MP are low. They're going to be in the wilderness for a long time: if you've got a diagram of the power structures in the Labour seniors, roll up that map, it will not be wanted these ten years.
If I were Cameron, I'd do something so heinous in the Spending Review that the traitors LibDems revolt against the coalition, force a confidence motion (Labour are so stupid they'd side with the LDs to vote it down) and have Ed Miliband fighting a general election, leading a bankrupt party, in late November. Return to office with a slim majority, I'd then kick back and watch the Labour Party descend into fratricidal chaos, knowing the Tories are in for twenty years.