I think you never will be able "to BUY STUFF only with Bitcoin". The word "only" is not correct.
In short: people have feelings, Bitcoin alone is boring, you can get different experiences with different coins and communities.
Coins are being implemented into market, payment gateways, gift cards, exchange services, multi wallets.
Altcoins are backed by Bitcoin and Litecoin.
Technically for altcoin holders there is no difference to buy with Bitcoin or with some altcoins: you must send coins to an address and that is all.
If you hold altcoins, it is easier to send altcoins than to exchange them to BTC first.
Even if everybody can buy coffee or dessert in McDonald's it does not mean that McDonald's will crowd out coffee houses or even other fast food services. I hope Bitcoin will be a leader of cryptocurrency like McDonald's is a leader of fast food industry, but some other altcoins will not disappear, there are many coffee houses, not only Starbucks.
https://bitcointalk.org/index.php?topic=1133912.0The more Bitcoin will become solid, the more altcoins will lose reputation.
Maybe not now, not this year, but the next year, when STILL only Bitcoin will be really useable, when you will be able to BUY STUFF only with Bitcoin, altcoins will begin to disappear, probably with a big boom altogether in a couple of weeks.