Just re-watching the TOS movies and I without reading whether this is confirmed or not, I think the main motif from II to VI, it's solving the Klingon CKonflict.
By the time we see TNG, the Klingons are allies and Worf's presence is there to show that many wounds have been healed. So, the next logical step was to follow with the Romulans. The first season ends with the re-introduction of the Romulans and they make it a big deal. Through the rest of seasons, we see the Federation approach them and being a bit more accepting and open minded and if I didn't know anything about ST, I'd think Unification was the checking point to remind us that something big about the Romulans is coming. The Chase also hints one of them talking that, maybe in the future, they can be in better terms.
Maybe Generations was the movie to do this. Kirk was there to link TNG with TOS and make the Federation start the peace negotiations with the R. Empire. After that, with First Contact, we have the new enemies, the Borg.
This logical step was not followed, though.
Instead, they destroyed Romulus. After they also destroyed Vulcan.
See, STVI handles this plot very well: the destruction of Praxis isn't there for shock value. The Klingons destroyed the moon, and before their whole race dies, they have no other choice but to wave their white flag and open communication with the Federation. We see how both the Federation and the Klingon Empire are reluctant, that's the main plot. We see the real result in TNG (even though TNG was released before STVI.)
But in NuTrek, the destruction of Vulcan and Romulus is retarded. And how it was handled in Picard is even worse.