if bitcoin plunges to $0 will altcoins be affected? If the value of bitcoin where to drop significantly to amost no value would altcoins be affected ?

i guess they would because usually we convert to BTC before cashing out into fiat but wanted other opinion.
Yes, for the reason you gave firstly (because most exchanges only have BTC/alt markets not alt/fiat markets), but also because Bitcoin is the first, most well known, and longest standing currency. If its value dropped to near 0, that would be a reflection of public loss of faith in cryptocurrencies in general, not just bitcoin, Bitcoin basically represents all cryptocurrencies to laypeople and its death would likely coincide with the death of all other currencies.