Is there any mechanism built into the steemit blockchain to keep it fom becoming unprofitable for witnesses to run servers if the Steem price were to plunge.?
Nope, there is no such mechanism available.
Also, many users will most likely continue to host their nodes even if it becomes unprofitable. In order to make the nodes unprofitable, steem would have to dump down really really low. If something of that sort happened, there would probably be very little users active on the platform itself.
The users who are consuming content in the Steem platform bring value to it. In order for steem to plunge down below 10 cents again, these users would have to either become anti Steem or leave it altogether.
This is a highly unlikely situation. But of course this is possible. We don't know what will happen in the future. But if Steem went down that low, then Steem itself will be most likely abandoned before witnesses start to shut down their nodes.
Personally, I don't believe such plunge could happen in the foreseeable future. Maybe Steem could dump to 30 - 40 cents and again jump back up but I don't think it's going to go lower than that.
The top 21 nodes make a lot more money than other nodes. They won't become unprofitable easily.