That means if the dominance of bitcoin decreases in the market, lets say, the dominance of bitcoin is only about 40% of the total marketcap in the market. then effect of bitcoin is certainly not as big as before right? Bitcoin price movements may not affect the price of an altcoin market too much..
That's not right because Bitcoin is still the most widely traded cryptocurrency the amount of impact it does to altcoins won't change. Last year Bitcoin's dominance was less than 40% and we all saw how many billions of dollars were wiped because of Bitcoin's gradual decline. Kasabus is correct the dominance is not a good metric to determine how big can a cryptocurrency cause to other cryptocurrencies.