Existing Bitcoin holders spend their coins, this is the main effect of mainstream merchant adoption.
Why did these bitcoin holders buy coins in the first place? You are presuming a large demographic of people that considered bitcoins worth having when there was little merchant adoption but not worth having now that bitcoins are more widely accepted.
Besides, there's a far simpler explanation for the 2014 bear market: A correction to the 2013 bubble(s).
We can't prove that the price decline is triggered by mainstream merchant adoption with the merchants dumping bitcoin for dollars. I do not believe in this theory, because merchant adoption is increasing in general with some of the merchants keeping their bitcoin. Also I do not think that bitcoiners waited for more merchants to cash out (they could have done this earlier if they'd wanted to), instead I consider the majority of early bitcoiners being hardcore holders which rebuy Bitcoin any time they spend them. Why? Because they adopted Bitcoin to escape from the shortcomings of the current monetary system.
So it's reasonable to assume that the decline in price stems from an ordinary correction of the epic rise of 2013.
I think higher merchant adoption is causing somewhat of our current price decline. I also believe it will be short term. The reason some people may have not sold their bitcoin would be they would need to give up privacy in order to do so (or risk getting scammed on places like LBC or the currency exchange section). I would argue that some people with, say 50
BTC might decide they want to "sell" 2
BTC to overstock (who would end up selling 1.8
BTC of this) in exchange for some goods on their website.
I would argue that over longer periods of time that merchants will start to discount prices for people paying in bitcoin which will result in more people buying bitcoin in order to take advantage of these discounts