How many bank accounts does an average person have? Even for the most extreme cases, we're looking at the low hundreds of statements annually, at the maximum. If you're not an average person but instead a business or an archivist, then you need a custom system anyway.

I'm really not trying to criticise or diminish the value of your system. All I'm saying is that even without an additional tag (or hybrid tag+hierarchy) overlay, a hierarchical system can be quite useful as long it's well thought-out by the user.