Volume is great for Ethereum. If you ignore speculative value, and long term savings value, Ethereum's value is pegged to the float required for its pure transactions. If they are transacting a billion dollars a week in ICOs, and the typical ICO user holds for 6 months, then that puts a floor on ETH at $24B. That's $24 billion of real value. As valuable as $24B of petroleum, so long as there keep being ICOs... even if none of them are worth anything.
But the Ethereum based tokens that do succeed will be a net benefit for ETH, since ETH is required for the smart contracts that govern those tokens to operate.