Have you seen a Star Destroyer? Those things are pretty grungy and grey; they don't look anything like the porcelain-and-chrome space toilets the Grand Army of the Republic was flying around. The AT-ATs, Storm Trooper armor, the guns, they all looked like they'd been kicked around in the sand for years before they ever came on screen.
Compare the Tantive IV you mentioned, a ship belonging to the royal family of one of the most influential Core worlds other than Corcusant itself, with the Naboo Cruiser which is supposed to come from an insignificant backwater. The Prequels are just a lot cleaner and shinier period, which really doesn't jive with the "last days of the republic" feel they were going for.