Coinbase currently allows people to buy Ethereum and Litecoin with fiat. I say Coinbase first because it is by far the largest gateway into crypto at the moment. On top of that we are seeing many other exchanges allowing the purchase of numerous coins with fiat. While I agree that if Bitcoin were to crash it would severely affect the market, in all likelihood I think we would not see an immediate crash but a migration to various cryptos the operate at a better scale. Likely Ethereum, IOTA, or something similar would become the flagship, but again anything is possible.