I think Keys strength and his greatest weakness was a lack of real political ideology. He just wanted to be PM so was completely poll driven. If it was a hard decision and the polls said it would loose votes he wouldn't do it, but he had no real ideology driving a specific political aim.
That meant once National had captured the middle ground they would stay in power until they did something to alienate that middle ground. The danger for them now is that none of the candidates for leader actually give them an equivalent person so the middle ground will be open to competition in the next election.
Collins would alienate almost all of it, English carries the baggage of his past and allows the opposition to fall back on the farming/National stereotype, he to me, is also a way better deputy than he could ever be a leader. And Coleman well, just who is he and what has he done. His TV persona is not great and he comes across a bit preachy to me.
The best they could do out of the current crop available to them would be a Joyce/English combination but I don't think Joyce is really interested and he definitely isn't interested enough to scrap for it.
Having said all that their saving grace is that Labour has Little, and he is still virtually unelectable outside the union movement.