In fact, Bitcoin IS a threat to other Cryptocurrencies.  Every single time a new Cryptocurrency launches, people are wondering whether it is going to become 'the next Bitcoin'.  Bitcoin is not directly attacking any Cryptocurrency but considering its size and success, it is indirectly destroying any other competition with out having to do any thing at all.

Every time there is a bull or bear run, Bitcoin is the one they all talk about.  This is ignoring the other few shitcoins that pump to only become another buried altcoin in the massive graveyard of shitcoins that died very quickly.  If there is something an altcoin community is afraid of, it is that their altcoin may not do as well as Bitcoin does.