Not only is the cost of creating content is pretty significant, but if a lot of what they produce ends up being pretty crappy, eventually it could convince consumers to move to different platforms. For example, I know that I personally have soured on "Netflix Original" Movies. The last two or three I tried to watch (Bright, Spectral, How It ends) were pretty lame - lame enough that when I'm browsing Netflix and I see a movie that says "Netflix Original", I'm inclined to pass.
On the other hand, I feel like their Series have been pretty good. More often than not, I've enjoyed them. But if the quality of the Series took a dive, I'd probably just switch to Hulu (I already have Amazon Prime too).