The next week after the bitcoingold comes out, bitcoin will start to decline in price.
Perhaps the price of onion will skyrocket
That is true. People are stocking up BTC to get their share of BTCGold. After that, they will convert it back to different cryptos. The advantage, that some people would have perceived, is that in this frenzy for free BTCGold coins, the price of other cryptos will fall (like with DO). So they will get back more of their original crypto when they buy them back after 25-Oct. And as the market swings, the prices of other cryptos will rise to balance the demand and BTC will stabilize at a lower price. That ends up as a Win-Win!