If you are going to assume that price doubles due to reward halving or if your calculations assume that btc price goes up a lot to reduce the break even time, then it makes more sense for you to buy BTC today and wait for the price to go up. You will make more money that way. So as far as I'm concerned, I need to assume btc price stays constant or that it only increases enough to cancel out the increase in difficulty.
I was just adding to your assumptions because it may be relevant for some people. In the bitcoin world, anything measured in years is a gamble...
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See the post on this page explaining the DRM thing.
Any one want to wager on whether this comes true or not? 
Feel free to open a thread in the off-topic section to discuss and bet on that.