Once you've bailed out the banks, then what? You're going to bail them out again? (No, because they're now publicly owned). There was no reasonable scenario where that was not a one-off increase in the debt.

We were borrowing at 1% (basically free money because inflation was higher) and we might have spent that on growth policies, but George Osbourne thought this was a good way to reduce the size of the state, a political decision.