Are fire types becoming rarer in new generations?
Ehh. In the 4th gen they went a bit dead, but 5th gen they fixed it decently. You had to get Chimchar or Magmar in D/P to have a good fire pokemon. Gen 5 gives you plenty. You can get both pansear, who I don't really like, or darumaka, who I really like, relatively early in the game in b/w. Then in b2/w2 they give you even more choices by bringing in older ones. Plus there is litwick and heatmor. Nothing amounts to the amount in gen 1, but I'm not complaining.