There about about 10.5 million BTC already mined, with another 7200 trickling in per day, until two weeks from now when it will be 3600 per day. 3600 per day is something like 0.00034% of the total supply. So supply is mostly fixed, leaving it up to demand to change the value of BTC.
Some people point out that some miners dump their coins the moment they get them, but even if we assume that every miner does that, that would be 3600 coins per day dumped onto the exchanges, out of roughly 30-40k transactions per day. So miners are less than 10% of the coin exchanges. That's still not enough to move the market enough to double the price.
One very big problem is that wealth is extremely concentrated so that if one of the major holders of BTC (such as Pirate) sells a few tens of thousands of BTC on a particular day, it would flood the market. More likely they would try to sell only a few thousand per day to as not to spook the market. But this leads to price stagnation or even a fall in price.
If miners holding coins do the same thing, it would lead to lower prices as well. And ASIC miners with such high upfront costs will be under more pressure to sell coins the moment they get them, unlike casual miners who are just doing it as a side hobby. As ASICs rise, GPU mining will fall as a percentage of total mining, so you would expect that over time, the percentage of miners who insta-sell will rise, and therefore there will be more insta-selling by miners as a percentage of mined coins in the future.
If BOTH large stakeholders and miners do the same thing and dump coins, we'd see a steady march towards zero. Eventually they'd run out of coins, but by then the market may be so depressed and demand be so feeble for BTC that we'd be back so $2 or less per coin. Conversely, if neither dump any coins at all, we'd probably see a slow increase in the value of coins over time, but I think that's highly unrealistic given the ASIC insta-sell pressure and people like Pirate who are looking to cash out.
If a rival to BTC gets stronger, we also see price risks down.
Personally I think price will remain steady on the day rewards halve, as that event has been priced into the market already. I think we'll continue our current descent in prices as some major holders and miners sell off. I don't think we'll see prices hit $15 again for a very long time.