That's a bit of a fallacious argument. The developers have already allowed all content to be done as either 8m or 16m with the exact same rewards. It wouldn't be a snowball effect to ask for an 8m version of ops grade content because all other ops content already *has* 8m versions. It's not asking for something that isn't already available for absolutely everything else for endgame, which *could* be interpreted as a snowball effect were it implemented.
The best reason that I can come up with would be that it would require either separate coding to vary the strength depending on the number of players currently in combat (such that, if there are only 8 stacks of the debuff active, the boss is only set up for 8m whereas if there are more than 8, he converts to 16m) or a separate click with the different mechanics, which, regardless of how you do it, means some degree of work.
Personally, I would *love* it if the devs would actually create an 8m version of NP, mainly because it's a bit unfair to restrict a specific piece of endgame content to the exclusive domain of those guilds that can field 16 people with regularity. For those of us that really don't enjoy the hassle of dealing with 16 people at a time, it's a truly rare occasion when you're capable of doing NP; allowing for an 8m version would mean that more than just a tiny minority of players actually end up doing that piece of content.