More money coming into BTC will be very good for alts too at the end of the day. Even if the big money can only buy BTC through the ETFs and they can't buy anything else, the higher BTC prices and lower alt prices in the short term will encourage regular holders of BTC like you and me to trade into alts, which will then cause their prices to rise too.

As long as money is coming into the crypto market, all good coins will rise - a rising tide lifts all ships, as the saying goes.