I would argue that the union as formed, has not been preserved. What we have now is less than the Constitutional Republic as founded, due to an all powerful central government. Which has known little restraint since the war.
Personally, I'm not sure the republic envisioned by the apologists of the south ever really existed. Kind of like how conservative Presbyterians have idolized the American Puritan community. I suspect that the truth in both cases is different than what is envisioned by their proponents in their romanticized, utopian views.
That said, I'm not a fan of a strong centralized government. IMO, the issues behind the Civil War are complicated. Black and white thinking is probably not applicable in a lot of cases. I don't assume someone with sympathies to the confederate cause is a racist nor should someone with sympathies to the union cause be assumed to be a proponent of centralized government. There were good men and women on both sides of the conflict and there were good and bad causes on both sides. That is probably why it is such a tragedy.