When the altcoin bubble bursts, all ICO dreamers will be desperate to convert their tokens back to BTC and ETH. Then they'll sell them for fiat, but that will be maybe 10% of what they once brought into these coins when they first had to pay for the ICOs.
Even if Bitcoin and Ethereum are affected when the bubble pops, I can only see them emerge stronger in the long term. Are Bitcoin and Ethereum the best hedge against shitcoins?
ETH is considered shitcoin too