Even if the concept doesn't die off, the technology called Blockchain might remain and be revived, but if Bitcoin crashes and vanishes, altcoins won't be able to survive because people would sell their altcoins as a first when they see Bitcoin crashing badly since the whole market knows that altcoins move along with Bitcoin. When it goes up, they go up as well, and when it crashes, they can't stay profitable. So, when something like that starts happening, altcoins will be the first ones to crash.
However, once the dust settles, people and developers will obviously start working on cryptocurrencies again, and there will be a new beginning for the industry. However, it won't be the same this time around if there is no Bitcoin, most people might not have any interest in cryptocurrencies after that.
This is true, blockchain is so important but bitcoin is even more so. Think of blockchain as the sea, and bitcoin as the lighthouse, people may still have the sea but without the lighthouse they would be too afraid to voyage through the dark waters with a fear that they may crash.
This is why we need to end up with something that would be a lot better on the long term, it should benefit them one way or another. I know that it is going to cost us a lot to just keep bitcoin up for that reason, but lucky for us we also love bitcoin for what it is itself without the need to say anything about the altcoins coming along with it. It is like a whale that feeds other fish. We should love them all, and think of them as one part of a bigger whole.