In a sense, enabling code like this on the site is kind of a matter of if Steemit wants to support that "convenience" or system. But overall, there's no way to stop me from saying, "For 5 SBD I'll put your banner on the bottom of my next post... it'll be there forever!" Big influencers will no doubt do such a thing, just as they're paid to write a tweet about an ICO or post a picture wearing a certain piece of clothing on Instagram. This would just allow for a more dynamic ad inventory that stays more relevant over time, and democratize it so smaller users could benefit as well without the need or means to broker direct deals for a few pennies!

As to the system... I can give my take. Steemit Inc. will always have control over Steemit.com, all it's interface, upgrades, etc. That's their private site. But they have no control over any of the other Steem front ends or how the Steem blockchain is used in general.

Steemit Inc. is however the primary developer of the blockchain right now. It's their mission and they have a big interest because they own a substantial amount of the originating Steem. However, anyone can create new code and features for the blockchain, but it has to be agreed to and ratified by a large number of the elected witnesses.

So the best comparison is to politics. Yes we can suggest blockchain changes and features, I've even seen an interesting recent post by someone talking about hiring a large and well known development team to tackle some Steem issues independent of Steemit Inc. and work direct with the witnesses. (Wish I could remember who wrote about it...)But to implement those changes you'd also have to make sure you're voting for witnesses that have a similar vision of the future and would implement them. Here too, since Steemit Inc. has a large stake, they can heavily influence the elected witnesses right now and steer things in a direction of their choosing. But ultimately, if Steemit Inc. disappeared tomorrow, Steem would go on. There are online roleplaying games I played 15 years ago, long since discontinued by their publishers that have passionate fanbases running servers they set up themselves and keeping them alive. I can't imagine witnesses and the community wouldn't step in to do the same for Steem! Despite Steemit Inc's clout, the Steem blockchain itself is not their property anymore, it does belong to and is controlled by all stake holders.