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.
Do I misunderstand or are you sprouting out bs scenarios?
The near-conformity rule: best parents must be selected only among those parents whose witness list differs from the childs witness list by no more than one mutation. This rule ensures that witness lists of neighboring units on the MC are similar enough, therefore their histories mostly agree with one another. The parents whose witness list differs by 0 or 1 mutation will be called compatible (with the unit that includes them directly), while the others are incompatible. Incompatible parents are still permitted, but they have no chance of becoming best parent. If there are no compatible potential parents among childless units (an attacker could flood the network with his units that carry a radically different witness list), one should select parents from older units.
The above means that each unit must list its witnesses so that they can be compared. We require that the number of witnesses is exactly 12. This number 12 was selected because:
it is sufficiently large to protect against the occasional failures of a few witnesses (they might prove dishonest, or be hacked, or go offline for a long time, or lose their private keys and go offline forever);
it is sufficiently small that humans can keep track of all the witnesses to know who is who and change the list when necessary;
the one allowed mutation is sufficiently small compared with the 11 unchanged witnesses. In case a user thinks that any of the witnesses has lost his credibility, or there are just better candidates, the user can replace the witness with a new witness in his list, bearing in mind that his witness list may not differ from that of other units by more than one position. This means that any changes can happen only gradually, and a general consensus is required for a change bigger than one position.