Andreas Antonopolus has stated in his presentations about Bitcoin, that it was in fact looking to establish itself as a global reserve currency. That seems to have happened already on some level, at least in the world of cryptocurrency. As you have pointed out, if you are looking to buy altcoins, you must first purchase Bitcoin to trade for them.

I believe that buying groceries, cups of coffee etc. will be the function of altcoins. It won't happen overnight, but the altcoin community that works to demystify cryptos and to simplify their use for the larger population will be the coin(s) that will be adopted for everyday trade.