It's true, though. Sonic was always made to really shine in replays, as you master the levels and find new, cooler feeling ways to beat them. Adventure 2 incentivizes this the best with the extra missions that all serve to help you get really good at the mechanics and levels, and then the A Ranks to incentivize you to really master everything. Then there are good unlockables spaced properly apart to keep you trying to get all those missions and A Ranks. The only other game with unlockables as good is Generations, and it's too easy in general.
Also, no other game in the series has had a Ranking system that works as well, with as many factors contributing to your score in meaningful ways. It's not just speed, but also chaining enemies and item boxes together, for example. That might sound counterintuitive, to have Sonic not be all about speed, but it makes the game way more fun, and makes it feel way more satisfying than just deliberately skipping as much of the level as possible. There are a couple times when you actually want to take a longer route to get a higher score, but it feels satisfying in practice. Granted, not every mission type does this, some are just about speed. Those ones aren't as good.
The Extra Mode has, on the surface, five times as much content as Story Mode. But it's not really like that, because yes, not all the missions are as much fun or content as the first missions. But there is also getting the A Ranks. That adds way more content to the game overall. The lack of replaying levels in Story Mode means you miss really trying to get the A Ranks. Again, Sonic has always been about replayability, and the Ranking system is where you can learn just how well the levels were designed for that purpose. It goes for other games by the same team, too, like NiGHTS. NiGHTS' story mode is just an excuse plot. The real point is replaying levels over and over for better scores. Sonic Adventure 2 is the same thing.