I don't think this is a bad thing in itself. Most of this market cap is first of all just a speculative bubble because the price is not really the price of "money" (the bitcoin and other coin USAGE as a means of exchange doesn't justify such a market cap) and is also not really the price of "store of value" (most bitcoin holders don't hold bitcoin as "store of value" - that is, to get out about the SAME value years later, but to get out MUCH MORE value without there being an equivalent production of value standing over it - so most of bitcoin's market cap at this moment is still coming from "greater fools" theory rather than "same value").
By this I do not mean that bitcoin's price will crash. Absolutely not. It might well be that bitcoin gets used one day are genuine money and that its monetary market cap will even be higher, but for the moment, the market cap is essentially speculation and doesn't come from the normal monetary value it should have as a means of exchange.
So if this market cap lowers a lot due to what happened to the DAO it won't hurt bitcoin as a means of payment (it has largely enough market cap remaining to do all the purchasing of goods and services one is doing with it right now, which isn't a lot).
On the other hand, a wake-up call was necessary. It is necessary to see what happens when the block chain paradigm fails, such as happened to ethereum, and when a chain forks genuinely. That such a block chain failure would occur one day now seems obvious (after the fact) and it is good to be aware of it, also for bitcoin.
That a few billion of hot air speculation got burned doing so is not so much of a problem I'd say. Crypto is testing its limits, its behavioral features, and we need to know more about that before going further.
Also, my personal hope is that one will wake up about this monumental problem in bitcoin, which is traceability and the potential colouring of coins, as ethereum is now also experiencing.