The newer CPUs and GPUs coming out are shifting to higher cache/vram. When new technology comes out, it will definitely help your coin to be the most profitable for those who are willing to spend more capital on new products. Otherwise, people who have had their GPUs for years will just simply mine and dump your coin, and they won't even have to worry about ROI at this point...
Convinced yet?
Since most GPUs from high end to low end are pretty much on equal footing at NF15+, with speed more or less dictated by their amount of RAM there is less need to perpetuate the arms race of newer better faster more expensive hardware to remain an effective miner. Since there is not a continued requirement to invest $ into new hardware there is also less need to dump mined coins to payoff the capital costs.
Some continued arguments for Kracko:
I think the equality in mining higher NFactor chacha is a very marketable feature. Both for hardware costs and power usage. At higher N the all GPUs are drawing less power as NF increases, but this is particularly noticed on the lower end cards. My entire 6x R7-240 4GB system currently pulls only 150w at the wall (I believe it was 180w at NF15). At NF15+ this system is hashing just as fast as a 6x R9-290X system for a much less ridiculous capital cost and runs quiet and cool as a cucumber.
I was recommending NF15 over 16 since there are many more people who have 2GB GPUs in their computers (or mining systems) than 4GB. At N15 these are still functioning at decent efficiency. At NF16+ the 2GB cards take a big hit in mining effectiveness so there would be many less people willing to mine. Since I have mostly 4GB cards I am making this recommendation not for the benefit of myself but what I think is potentially most beneficial to UTC. N16 has been treating me very well.
So at NF15 you have a unique currency with 1) the most egalitarian mining hardware requirements of any coin (ie almost anyone with a PC can mine it), 2) has among the lowest, if not the lowest, power usage requirement for GPU mining of any coin and 3) remains asic resistant and 4) is botnet resistant since it remains far more efficiently mined by GPU and not CPU. That combination of features is the golden egg in my view that so far has eluded others.
All that said NF14 would still be ok in my view...just not as perfect. But as you stated you are also working toward lite clients to function on android and other weak systems so if that goal is not obtainable at NF15-16 then that is a valid enough point.