Y'know, it was pretty much implied by the two trilogies ( horrible as the second one was ) and even shown during the Clone Wars that clone troopers were originally meant as an emergency measure, rather than a permanent slave-labour class to be mass-produced for the convenience of regular citizens of the Republic/Empire ( the role of slaves being performed by droids and/or certain nonhuman races under tthe Empire ). Basically, the idea was "we need a temporary army, and we need it fast, to buy us time to whip some actual citizens into an armed force."
When you look at the height discrepancies between individual Stormtroopers in Star Wars and Empire, and compare the few non-Stormtrooper soldiers that we see in the original trilogy, it becomes pretty obvious that clones went from being the whole of the army in AotC to being a minority of forces in SW, and really, AotC established that they grew at an accelerated rate when compared to their DNA donor, but some production materials from RotS showed that the accelerated aging didn't stop when the clones reached maturity, so they only got half of the normal service life that a human would, which is already less time than most other races get, so it just makes sense to scale back the clones when they are no longer a necessity.
As far as "Black Stormtrooper" goes, though, when I saw that trailer, my first thought was "Oh, look! The galaxy got it's third black person!" I'm rather interested in seeing the humans of Star Wars become a little more diverse, and am kinda glad they chose the "make new characters that aren't white" route rather than the typical Hollywood "Let's recast this established character as a black guy, then tell people it's a reboot, and if they complain, we'll just call them racist."
Final note: Star Wars, as a franchise, is actually one of the few places I wouldn't complain about Cinderella Casting* choices for families, either, since we're talking about a galaxy where there's already an entire species that comes in every colour of the rainbow, and skin colour seems to be like the patterns on calico cats for them, in that they seem to be determined by some other factor than DNA.
* Note: I'm talking about the musical from the 90's that had Brandy as Cinderella, some Asian man as the Prince, a white guy as his father, the King, and Whoopi Goldberg as his mother, the Queen. That royal family makes no real sense to me to this day.
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