Darling was a temporary appointment; no one believes that Brown will keep him in post after the next election which explains why some young Brownites were so keen on an early election. But to move Darling before polling day would be a huge risk. First of all, it would call into question Brown’s judgement in appointing him in the first place. Second, it would add considerably to the feeling that this is a government on its last legs. Finally, there is no guarantee that Darling would go quietly.
For the Prime Minister sacking Darling would—in the Yes Minister sense of the word—be the courageous thing to do. It is hard to see Brown being this reckless.