If bitcoin goes mainstream, it would definitely affect other crypto currencies too, but very little. Others may not necessarily go mainstream. The reason why I say this is since:

1) Bitcoin alone has enough liquidity to handle $100,000/coin or even more and this would not affect its usability as people can still use the smallest denomination, a satoshi. So the finite supply is not a concern at even $1m/coin.

2) Global acceptance will have an important part in making it go mainstream. Right now, online/office vendors have started accepting bitcoins but thats too little, just like how paypal was in its initial days and compared to this, very very less business are accepting other crypto currencies. I know a lot of business who plan to start accepting crypto currencies soon and all of them mean bitcoin. So, why would other crypto currencies go mainstream when you can't actually use them around the Internet?

3) No one would mind buying bitcoins at a higher price, eliminating the need for other crypto currencies. This is so because all vendors peg the bitcoin with USD or some other currency. So if bitcoin prices soar, it really doesn't affect the new buyer much.