I guess, never.
Bitcoin has price because someone is willing to pay for it. Right now, because its price is going up, and Bitcoin is in short supply, people are willing to pay much money for it. And the ever-increasing demand for Bitcoin will keep driving the price up until people don't want to buy or hold it anymore. Would some one like to buy BTC for $100,000? Just to get $100,100 lately?
At that point, by the way, unless Bitcoin has become widely accepted as a legal means of exchange (as "money"), the price of BTC will collapse someday.
or, you know, other cryptocurrencies will pop up.. not to take over BTC though.
This. There already are cryptos that are far more convient for day to day trade. You also don´t use gold for buying food and paying bills, right? That said Btc could very well reach 100k by 2017, if current rate of growth holds. It would only take fraction of worlds wealth.