What pressure prevents alternate currencies from completely diluting bitcoin?
That pressure is purely market opinion of each currency as it comes into the public view.

F.A. Hayek argues that competition among currencies is beneficial for the economy and a useful means for fighting inflation of currencies. See Choice in Currencies.

Bitcoin by the very definition of its protocol is, in the long term, a deflationary currency. This is a fundamental quality that the introduction and rise of competing currencies - many of which will be modeled after Bitcoin - cannot change. Bitcoin cannot be diluted, because bitcoins cannot be arbitrarily added to circulation on a whim. Those who may have this power are under tremendous public scrutiny and would have a gargantuan task of persuasion ahead of them, should they choose to attempt to sway Bitcoin users' opinion.

Competition among the various alternative cryptocurrencies remains a topic of considerable debate, but the market cap chart to which you linked in your question comment indicates that someone finds value in these alternative currencies based on their merits.