Back in the 1920's when that great depression struck, many banks folded, and people who had money in banks ended up with nothing.
No matter for what reason the banks folded depositors were the ones left holding the empty bag.
Kinda like what is happening in the various Bitcoin exchanges. No matter if it's stupidity, lack of security, or malice, it's the depositors (whoever parked their Bitcoins in that exchange) ended up losing it all.
Well ... back to the 1920's.
When the banks folded, did people abandon the greenbacks ? Yes or no ?
Same situation here ... The fact that exchanges vanishing into thin air doesn't render Bitcoins invalid.
True, some of the "rules" are flawed ( I kinda have a sense something is amissed ever since Bitcoin came out, back in 2009, but I just couldn't pin-point what is wrong with it, but thanks to those scientists at least now I know, but I digress ... ) and they may need to be changed ( ... as been pointed out, the implementation of the necessary rule change may turn out to be very hard ... ) but all in all, the system of Bitcoin, at least, for the concept of it, is still as valid as ever.
Many people are digging at Bitcoin, trying their best to make it sounds as if it's something uncertain, something ephemeral, something "flash in the pan" but if we are to look at the alternative to Bitcoin, ie, the FIAT MONEY SYSTEM, it too has been damaged beyond repair --- as so much money was created out of thin air, which means, the value of the fiat money is no longer valid.