Because you looked at the polls and the betting markets and you thought you knew better. Once again, you were wrong. Remember a guy called Kerry?
It is not the Bank's debt; it is the UK government's. You can read the DMO contracts if you like. And the debt does not have a monetary purpose: it is issued to finance government spending.
You're right, sterlingisation can't be blocked, but full-on currency union certainly can. I don't think anyone in the Yes camp gets that no-one in Westminster is interested in entering into a currency union - not with Europe, and not with Scotland either. What's in it for them?
"Protozoa are small, and bacteria are small, but viruses are smaller than the both put together."