First of all, there is no way it will saturate and that should be certain with today's technology.

Now with respect to the future of Steem Dollars (SDs) it can be explained or pronosticated by understanding what value and money is. I think that the sole concept of creating money out of content is a natural thing. Social media platforms like FB are also making money but in an indirect way (profiting from ads). Steemit content makes money more directly, not from ads but from the simple fact that "content = value" and everytime value is created there is a counterpart (analogous to matter and anti-matter ) that represents it, in this case Steem Dollars. So the future of this kind of Social Media Platform shouldn't be so uncertain.