Yes and no. I think it's a matter of interpretation. A witness who doesn't post frequently enough is, technically, useless for the network integrity and to assist normal nodes to have their transactions validated as fast as possible. So, that means this lazy witness is potentially a bad witness and should be exclude at term. A temporary lack of posting could be obviously tolerated when there are technical reasons but it should be limited in time.
So, in my opinion, a witness has a mandatory behavior to be selected/elected as a good witness.
A witness is incentivized to post frequently enough. This is not the same as having mandatory behavior.
Technically, if 7 witnesses stop posting, network halts. Due to a law enforcement, they could have to do it, who knows... Witnesses have to be very distributed over the world.
I think almost all users/all potential users don't care about witnesses selection, like me. They will accept whatever byteball foundation say. Or better said, what byteball.org/bb says regarding to witnesses list choices.
As non-official witness, you definitely don't care about the incentives because you don't receive them.
No, network will not halt if 7 witnesses are down. But normal nodes will take more time to define their best chain until the genesis unit.
That's why, all of people who care about Byteball network should honnestly consider to run a non official witness and set it as permuted witness for our own wallet and give our non official witness address to our friends.
Plan the worst to get the best.
I am not saying that is not going to work, it is simply that I would call "distributed trust" instead of "decentralized trust", because anyone can easily see 12 witness as 12 central points.
Byteball is a distributed database with open access and distributed consensus amongst 12 witnesses.