Are you even following the market trends ?
Bitcoin is now $8000 and you are still looking for the right time to come . Invest now or forget it tomorrow because it might grow more than $8000 soon .
I don't think the person who suggested you that "altcoins may drop bitcoin" knows a thing about market . Do your research , you will know bitcoin is dominating so much that all the altcoins are going red and going through the worst phase at the moment . If some other altcoin comes up with a good price then bitcoin price may fall , but at the present time bitcoin is the king !