Yes a single coin should not determine the fate of the market. Whenever bitcoin goes up market goes up and whenever bitcoin goes down it also goes down. But when a coin like btc has 54% market dominance it is bound to affect all others.
Everybody talks about bitcoin, in the media, everywhere and people first hear about bitcoin.
So, bitcoin became main entry point for many new users.
When they join bitcoin and learn more about crypto market, they start to invest in other markets.
I don't think that such situation will change soon.