To gain any insight into the potential market cap of steem, you must first examine the landscape which it targets: media. Looking at the 700$ steem as a possibility - steem would need to be a major player in the industry, along the likes of The New York Times.
If in a singular fashion, the steem blockchain became the primary publisher on the planet - the multi billion dollar markets are reasonable. I do not think that steem will achieve this alone. There will be many content blockchains with their own uniqueness.
Another thing to consider about market cap is that blockchain technology enables a bunch of other things that traditional media does not. User accounts with reputation (which has value in itself), being able to follow/reward individual content creators, not being tied to a single publishing house.
On the other hand some costs of having a public blockchain include the inability for steem-holders to profit from data mining user's data (since it's public and everyone can data mine it), publishing and curation tools are still in their infancy (for those who want to publish a controlled message), and the ability to publish with a centralized like model (still desirable as many many people are accustomed to the format).
Great response. I feel there will be many different chains along a spectrum of user permissions and oversight.
now that's an argument i can get behind!! thanks for the comment.