User blog comment:B214/Volume 22 spoilers/@comment-29778526-20170126060633/@comment-9595671-20170126220408

I do think they rotate, that's probably how some of the group was able to talk with Ajuka and Issei in Volume 22, it was simply their resting period.

And while they can't destroy it, they can still weaken it by fighting it, I think due to how it's body[s] (I forgot how many were out when they used it) were locked in place when it ejected it's core, it literally is fighting one against many.

So the sealing likely doesn't just keep Trihexa from just rampaging to freedom, all the years it's sealed they'll be weakening it while the outside world becomes stronger, and Ophis could regain the power she's lost that is enough to kill it.

Rose's seal could be how they avoid deaths, to briefly lock it's movements temporarily while they recover, if too many of them died then Trihexa could free itself because the barrier's all that would be left between it's freedom.