To be fair, from that perspective, I'm not really sure we would actually have Umbreon since one could say it was practically made for the introduction of the Dark type. Where as with Steelix, they could have placed it under Rock/Ground if need be. That being said, the whole purpose of Dark and Steel being introduced back in Gen II in the first place was because the type system was off balance and it needed to be better rounded off going into another generation. Should they introduce another type, they would have to go through the type system once again and make sure it remains balanced.Here's another way to look at it. What type would you make Umbreon if the dark type never existed? What type would you make Steelix if steel never existed.
I disagree with the argument that says the lack of newer types limits GameFreak's creativity when it comes to bringing in new Pokémon. You can't really say that when there's still a number of type combinations they have yet to use.