There are a lot of altcoins out there and they can have a negative impact on bitcoin as everything invested into an altcoin could have been invested into bitcoin. It is good to have choice in coins but it does not help when they don't really bring anything new to the table.
The growing number of altcoins that been made can truly affect to the existence of bitcoin. Aside from the fact that it may become one of the rivalry of bitcoin in the market this might be also one of the reason for the confusion of bitcoin existence if it would continue to materialize or it would stop from being on top.