Bitcoin will grow no matter how much a certain altcoin is being pumped. As with all altcoins, they will get dumped to the ground at some point. Ethereum is definitely not an exception. New money will be attracted by the volatile nature of Bitcoin. The market takes care of itself nicely.
I agree. Due to the network effect (advantage of being first coin in the block) Bitcoin is independent from the other altcoins, and other coins depend on the price of Bitcoin.
Some people want to buy altcoin, but
there is no altcoin/US dollar market, so they have to buy bitcoin first.
For ETH/USD and LTC/USD there's Bitfinex, Kraken and Poloniex. Poloniex does other USD currency pairs as well.