NWOBHM was not a sub-genre. An era, a movement maybe, so it didn't died, simply evolved. Some bands stayed (Iron Maiden, Saxon, Venom...) continuously, some (Jaguar, Quartz, Battleaxe...) occasionally, and many either passed away, and at least one change dramatically (Def Leppard).
Many bands try today to capture the old feeling, like fashion does, likewise music. . Look all the retro bands playing in the 70's feeling/vibe, and thrash bands trying to sound like the ones in the Bay area at the end of the 80's. I see no fault, nor problem.