There's no guarantee that the money which was pumped into ICOs would have been invested in Bitcoin, if ICOs didn't exist. Some people were out for a quick buck and preferred gambling with different altcoins which only offered hype in the hope that one would give them a decent ROI quickly, rather than trusting the underlying technology of Bitcoin.

Competition exists in different markets and is actually necessary for stability. Altcoins may pump up and die off subsequently, but there would always be more which are created over time, so bitcoin's dominance would always be tested, the fact that it has survived through all the previous hype/dump periods suggests that it is reliable