Steem inc. develops the blockchain. It's a very crucial roll. I don't know if anyone can take over. I doubt it on the short run. I was happy to hear all the core developers are still there. I hope they can refocus.
Witnesses just verify the code and changes before they go live and run their servers.
Ah, okay, that makes sense. Are there witnesses that would be willing to step into some of those roles? Just a thought. It seems like the witnesses are very active and involved in the health and well being of the blockchain. They would have a vested interest to make sure it succeeds if they had the skills to do a little work in their free time.