Out of all the games that came out after R/B/Y, G/S/C probably had the most older Pokemon that you could legitimately catch. There were only a few that you couldn't, such as the old legendaries and starters. G/S/C were clearly designed to allow nearly-comprehensive completion of the Pokedex with one version of the game. Now you have Black and White with their little three-town area in which you can catch a number of Pokemon that existed in previous generations that probably totals in the double-digits. R/B/Y, in contrast, didn't have any older Pokemon to go back to.
It's possible that G/S/C was actually more revolutionary to the Pokemon series than R/B/Y was. R/B/Y were fairly typical J-RPGs with managing a team, items, and techniques that characters can learn. The main gimmick was the cute monsters - which were still nothing new - and multiplayer connectivity. G/S/C introduced more things that RPGs had not seen before, such as creating new creatures out of nowhere (Day Care), connectivity with older games (R/B/Y), and the essential amalgmation of two games into one (two regions).
Maybe I'm getting too critical and pedantic, though, and should approach the problem from a more personal angle. While G/S/C introduced fewer new Pokemon than any other generation, I think the Pokemon designs were superior overall. For example, the Kanto starters are more classic and well-loved, to be sure, but they also looked more like each other and were all basically based on dragons. Also, some of the creatures like Raticate, Tauros, and Spearow just looked too much like real animals; their designs were not elemental or very creative at all.
One more gripe I have about R/B/Y is the messiness of the Special Attack/Special Defense system. I really think it worked much better when Game Freak decided to completely separate Special into two stats.
I don't think that R/B/Y is a bad game by any means, but I still prefer G/S/C as I believe the game, Pokemon, interface, music, etc. to be simply better designed. If anything, R/B/Y is the overrated one. Many people seem to give the sequels a lot of hate, but I really believe G/S/C to be better games.