For me i Believe the other way around, if bitcoin will continue to go a downhill trend so as other altcoind will follow in the long run. Yes intially i think the altcoin will rise in the fall of bitcoin because investors will seek another alternative investments but surely if the mother of all cryptocurrency will degrade i believe all the more the reason that other altcoins will follow the same lead.
Agree with you, I believe this market is build this much because of Bitcoin. so if bitcoin falls investors can think why they need to make them self at risk. why investors need to trust on altcoins if their lead was fallen.