Well hm.
Fact is miners live to make money, if there is no profit in mining people will stop doing it till mining is profitable. Right now there is a mad rush to buy .28w/gh mining gear (I'm not quite sure why) and probably a ton of .8-1.0w/gh equipment sitting on the sidelines.
But think about it: If the price of bitcoin suddenly dropped to 225, then the mining difficulty would drop to about 50 instead of where it is now. Because at a 25btc block reward, a difficulty 50 was enough to eek out a profit at 225 a bitcoin. Was the network any "less secure"? I don't really think so, your chances of breaking the chain were basically zero then as now.
So when the block reward halves, either one of three things will happen:
1) 50% of the miners will drop out.
2) The price of bitcoin will double
3) Fees will go sky high.
I think 2 is almost guaranteed, anyway this is bitcoin's anti-inflation promise: given same demand, the price will at least double every 4 years due to supply cut by half
The block reward halving is a 100% sure thing, the market will already price in it several months before, when it arrives, almost nothing will change, this has been proved by 2012 reward halving
Of course months following that event, some dumb miner which have done nothing will feel diminishing return thus start to shut down their rig, but the new miners with higher efficiency will be quickly deployed to compensate the loss in hash rate. Bitfury just announced that their new chip is 5x more efficient than the last generation, so a reward halving would still give them 2.5x increase in efficiency, resulting in higher difficulty
Second, many miners are not mining bitcoin for profit, they mine it for transfer value. Mining equipment company currently are not regulated by financial regulations, unknown capitals can enter bitcoin mining and leave it in form of fresh mined bitcoin without any trace. I guess the large bitcoin bubble in china and forever increasing hash rate regardless of exchange rate has something to do with large scale capital fleeing China. If you think about the nature of these mining operations, the fee is the least concern for them
So, difficulty will continuously rise due to fast changing mining tech, while total mining cost will still get higher and higher due to more capital entering mining and exchange to transfer value, these will all raise bitcoin's price, to more than enough compensate the loss in block subsidy