I thought Kanto was complete in the orginal GSC, Maybe I remember wrong? I don't thinkg HGSS has TONS of area GSC doesn't.... though... right?
Well, the version of Kanto you visited in GSC was pretty bare-bones. Almost all of the "dungeons" were removed - as far as I can remember, only Rock Tunnel and Diglett's Cave went untouched, and Mt. Moon was shrunk to a tenth of its original size. Otherwise, the caves had collapsed, the forests been cut down, the big buildings shut off or revamped, the safari zone closed - that list went on for a while. HGSS brought back Viridian Forest and Cerulean Cave, at least.
As for the lack of postgame content in ORAS, I take it as confirmation that there's another Gen. VI game on the horizon, and we might be getting it as early as next autumn. GameFreak's business model sort of hinges on the games having limited longevity, so they feel properly out-played by the time a new, fresh game comes in. With too much to do in the post-game, it makes the new games a lot harder to justify over the old ones, especially if it's within the same generation. Sales are increased by holding back features, releasing a little more per game as the generation progresses. Players are meant to be tired of the first games of the generation, so they'll buy the subsequent ones. With the next generation coming, the prospect of new Pokémon, moves, graphics, mechanics and regions trumps the replayability factor of the old games anyway. Cynical, yes, but effective as heck.
As such, Emerald had this big Battle Frontier whereas R/S had a simple tower. The Frontier could easily have been coded into R/S, but then it'd be harder to sell Emerald.
D/P had little once the Battle Zone was conquered. Platinum had a Frontier and Gym Leader rematches.
Strangely enough, there was no Frontier in BW2, but those games had the awesome Join Avenue setup, the PWT, the Shiny Charm, Medals and Pokéstar Studios. A full-fledged postgame experience, built to last unlike the rather small BW postgame.
In other words, with XY and ORAS both having rather boring postgames, I expect big things to come from the inevitable Kalos revisit. That game, or those games, will probably have content enough to keep us satisfied until Gen. VII is released (which I expect to be sometime in 2016/2017).