There is little reason for Noctua to rush out a next gen CPU air cooler. Even at this point in time, the current revision of NH15 is still one of the best. While competitors are catching up, they have not soundly beaten Noctua in terms of performance vs noise.
I agree that the D15/S is one tough cookie to match, but cost is still an important factor for many out there... and Noctua's user friendly Secu-firm kit won't be enough to turn their heads over that.
If those folks can get comparable performance for much less, I'm sure they'll be plenty happy.
For the price, NH-D15/S has like one competitor - Thermalright. The FUMA 2, is a runner up of sorts, but it starts to lag behind the big Ds with higher heat loads and going up from there - think 200w+?
The stock fans hold it back, but then the great low noise profile would be lost from swapping in stronger fans.
As you go down the stack from D15/S, the number of options from the competition opens up.
Looking at it that way... U12A, U14S, U12S, etc, are bloody expensive. D15/S is, ironically, the most cost effective of the stack.