The very form of government--a republican form--is what the founders determined was the best way to secure individual rights and liberties: better than a monarchy, better than pure democracy, way better than anarchy.
Now, if a State chooses to secede that is another matter.
I consider it a deficiency of our federal constitution that it does not spell out explicitly what is required for a legitimate secession. If 50%+1 of legislators or even voters wanted to secede for the purpose of re-instituting slavery among the racial minorities living in the State, or to ban RKBA, or to force a State sponsored religion on all residents, I think we would agree it would be a good thing for the feds to step in and protect the rights of the minority about to be oppressed.
But we should be able to imagine some orderly process by which a State could secede while guaranteeing the rights of its citizens, including, perhaps, super-majority thresholds, accepting some portion of the national debt/obligation, treaties for existing rights-of-way, and buying out those who wanted to relocate to a State remaining in the union.