The designers create nanos to meet their quotas of 1) nanocost/dmg over time, and 2) damage dealt per nanopoint. If the durations of our dots were doubled, that would throw everything out of whack, and most likely the damage of each tick would be halved. That is what the designers would
have to do in order to balance such a change.
We don't want that, I can assure you.
I'm honestly a little floored that nanocost is an issue people are addressing. At 220 with a nice nanodelta and close to maxed nanocost reduction, you can more or less chain-cast all 3 DoTs with nanospindles and genius2 for a very long time, with heals thrown in to boot. It gives incentive to gain better gear and level to 220.