A hub can do that, but if a unit (transaction) has more than 1 mutation its invalid. Only when 1 witness is replaced by others as well, when the new witness has posted enough stamps, the "stability point" can be moved forward, and then the next witness replaced.
In other words, if you replace one witness with yourself and I replace another witness with myself we'll split the network into two halves. There will be 50% of users behind your choice and 50% of users behind my choice. And of course, I will never accept you as a witness and you will never accept me. Looks like the consensus will diverge for indefinite period of time in this scenario.