So why Altcoins exist ??

[cryptodreem]

I think Bitcoin's price doesn't depend on ALT coins always. Every altcoins has some unique role and features and so the price moves up and down. But yes, bitcoin price moves a bit up or down since altcoins trading are done with BTC or ETH.

[Al-e_x]

I think your perception of bitcoin volatility is wrong, I understand your assumption that altcoin controls the volatility of bitcoin.

basically, a child cannot control the parent. Thus, the bitcoin being the standard altcoin value, altcoin value volatility has no effect on bitcoin.