Steemit is just the first phase in a much bigger plan...

https://steemit.com/steem/@dan/steemit-s-evil-plan-for-cryptocurrency-world-domination

Eventually the founders want Steem's value to become devoid of the success of Steemit, and for it to become a currency in it's own right. Check the link out, posted by one of the founders a month or so ago...

In answer to your question, no (at least, there is no ceiling based on user number etc). My point was short term looking. We need Steemit to be successful, to get ourselves to the next stage.