I'd be interested to know if there is a way but I haven't found one yet. And yes, practically it does make the other siege weapons an expensive waste of time and/or coin.

Rebuilding the walls is usually cheaper than building several Onglars and certainly quicker than waiting for other siege weapons.

I see it as an "End Game" minor flaw really. You should have to wait for Sapping points to be built and it should take the longest of any siege weaponry.