In particular, could the whole of Bitcoin be erased in one fell swoop, or would only a bit of it disappear - thus making the remaining BCs instantly more valuable
A very good question.
It shouldn't at this scale of problem, because AIUI, a blockchain currency uses distributed records so that the failure of any part of the system doesn't destroy the chain or the records. The Ethereum screwup made the BC inaccessible to the owners, possibly forever, rather than destroying them. From the perspective of all other currency holders they still exist. A bit like losing a fiver down the back of the sofa - you can't spend it if you can't find it, technically it is still part of the money supply. If the problem had "locked down" enough of a cryptocurrency to materially affect the trading market, then we might see rising values purely from lower liquidity, more so if the knowledge of the lockdown were public.
Having said all that, blockchain appears to be the twenty-teens equivalent of Tulip Mania, with about three times as many cryptocurrencies as their are national currencies, worldwide. It won't end well, and when it doesn't I'll rub my hands and say "Yer yunguns, yer wurstnt listen, I told yees, digital currencies 'er shite". And then I'll have another swig of cider, brush the dribble from my beard, and go back to sleep.