Okay, let it be half as much, but I still don't understand what you mean by halving the production (not reward), and how their earnings won't be two times less (shame on you!)...
Look, the blockchain reward is bitcoin production/distribution. There are new bitcoin coming into the miner's hands every single time they solve a block. Like with anything else (oil, coffee beans, gold, etc), when the production goes down and production prices remain, the price tends to go up. If we use a farmer as an example, say there's a drought... he sells less product, but he still has to make a living; sure, he may have to sell some of his grain stored up in a silo, but (assuming a purely local market) that's not going to affect the price all that much as it continues.
Let's assume the miners are going to have to sell their stashes... are their stashes 25BTC per block mined? Think about it, that is a huge number, and quite ludicrous. How long would their stashes hold the price down?
Look, here's a little timeline of what happened last time the halving took place. Jan-June 2012, prices were bouncing around $7 and $3 (mostly around $5-$6). Aug 2012, price goes up to $16, drops to $7 and bitcoin trades for a price under $9 one last time. Halving day 2012, price doesn't really move from $11-$13, the regular price. March 2013, bitcoin is sold for under $43 one last time.
I'm confident that whatever price bitcoin is at on Jan 2016, we're going to be trading it at double that price (at least) in march 2017. But only time will tell.