If Bitcoin, for whatever reason, disappeared, the whole cryptocurrency ecosystem would disappear as well, for the simple reason that cryptos would no longer be reliable. It would not be the failure of a coin, it would be the failure of an idea.
In 2013 that would definitely have been the case. these days i'm not so sure.
if bitcoin died from attacks or became too broken to continue everything else would take a huge hit and there'd be years in the doldrums but many of those coins would not be broken and the concept would live on through them. they would take the lessons that ended bitcoin on board and make sure it didn't happen to them.
bitcoin is the reference coin for all alts when it comes to trading and pricing. i don't believe an alt could ever take that place. i do believe a fully legit USD pegged token could.