Advancements do not just stop.
Well, they have
stopped in the real world, and for periods much longer than just a couple of centuries, too.
Generally, this would happen to a civilization in isolation. But civilizations clustered together into one complacent Federation would basically just amount to one big civilization, which would then exist in isolation and not enjoy a particularly stimulating set of challenges or threats.
The Romulans had the same issue. They broke apart from the Vulcans who underwent a large social change of global proportions at a time when Warp drive was first developed.
Intriguingly, Vulcans supposedly went interstellar about a thousand years before the Romulans left (the monastery of P'Jem thing). We don't know the exact chronological order of events - did Romulans first come to existence on planet Vulcan, then wait for Surak to be born before moving out in disgust, or did Surakism create the Romulans? We might even speculate that Romulans were the primary starfaring faction of Vulcan from the very beginning, and their "exodus" from Vulcan was more like a decision not to make weekend visits to the ancient homeworld any more... They'd then be an established interstellar power (rather than a ragtag group of refugees in failing ships, confused about what just happened) for three thousand years and still
only barely the match of Archer and pals in the 2150s.
Lots of possibilities there overall, with "everybody always keeps on inventing new things and these become household technologies in less than a decade" among the less interesting and evidently less common ones.