Because balance means more playstyle options for the players? Just because there was a lack of balance in older games, doesn't mean that the dex set-up was perfect. Why do you think people make fun of the amount of love water Pokemon get relative to other types? Also, what a laughable comment. You're saying that people are complacent and don't care about type diversity (which is a baseless assumption btw), and here you are forcing how you would approach the game down other people's throats. That is insanely ironic.
Sinnoh is absolutely not a cold region. Sinnoh has one winter city (Snowpoint) and mountains, and you generalize it as being a cold region. By that logic, Kalos is a cold region because it has Snowbelle City. Sinnoh as a region is actually widely varied in its temperature and climates. Sinnoh has varying climates: The Wetlands, Snow, Tropical (Battle Zone), and so on. Typically North Sinnoh is obviously cooler, but South Sinnoh is a warm area. That being said, you can't even use that to justify the lack of fire types.