The possibility of bitcoin price will decrease after bitcoin gold appears if you want to buy bitcoin not now because you will incur a loss so I suggest to buy altcoin and wait for the price back to normal and increase (only suggestion)
Yes that is the scenario i've see since many bitcoin gold holders are start to dump there coin and change it with fiat and same goes with the bitcoins since those past holders will sell there hodl share since the Fork hype is done and some of them earned some good profits already. And most likely this is the day for the alts to became more alive since the traders will came back to trade good with crypto's.