Bitcoin is still the base of all alts . To buy alts you need to buy bitcoin first
No you don't.
Bitfinex isn't a small exchange, all pairs exist in ETH now. Bittrex & Kraken have many ETH pairs.
Also I would be worried if the value of BTC was significantly based on that fact that it's bought to buy ALTs.
So far, when ALTs were going down, money was going back to BTC. Quite a problem if it goes to ETH instead.
Binance (which I think is a shitty exchange but everyone else seems to love) also has almost all of theirs paired with ETH as well.