Slow release of new Pokémon would be a game killer.
1). From a marketing aspect you build a lot of hype with a big rollout. Stretching it out for 15-30 weeks would not allow Pokémon Go to build any enthusiasm.
2). It also works against the game biome mechanic. Players could at least in theory get zero or 1 new Pokémon in a week on a slow release. It also hurts the biome mechanic in as much as if rare Pokémon is released late, which would happen if you released by pokedex number you would have months less time to catch it.
3). Interference with other people events that drive excitement from the player base and generate money for Niantic.
I fully expect to see all or nearly all of Gen 3 releases in the next 10 weeks.