marketplace share and lack of wide competition. When that competition day of reckoning comes and their shares start falling (I think it's impossible for them to remain on top at the same saturation rates) they're going to want to maximize the company's "worth" before it falls too far.
Consumers only have so much time and money they will spend on Streaming Service. (most don't have cable AND satellite) Disney will bloody their noses at least and Netflix will be limited on content to what they can produce or acquire overseas and people can drop Netflix faster than Netflix can deliver new must-have content.
With saturation shrinkage, it will be interesting to see how Netflix responds.