If investors keep switching to altcoins then obviously bitcoin will crash as more and more bitcoin leak towards the buying of altcoins. However this scenario in my opinion is extremely unlikely to happen for a number of reasons. Bitcoin is the first crypto currency which means that it will always have the trust and status unmatched by even major altcoins such as ethereum.

For example, there are much rarer metals than gold on the planet earth that have the thins needed to make a good currency or asset. However people had this trust in gold, much like crypto people have an innate trust in bitcoin. It's not all about high end technology but rather being good at what you do. In bitcoin case it is mainly the storage of wealth.