I think it's sort of interesting to look at the actual usage stats and see that there are 4 steel pokemon in the top 6 - Scizor, Ferrothorn, Jirachi, and Heatran, in that order - and then nothing else until Forretress down at 17, and Skarm down at 24. It's an interesting point to note that with Dragons and Steels especially it's only a very small number of pokemon with the actual typing that dominate. So to what degree is it because of the steel typing, and to what degree is it that these pokemon are simply really good? Let's be honest here, when you talk about a defensive steel type, are you thinking of anything other than Ferrothorn, Skarm, Forretress, or Jirachi? Maybe Heatran?
Other responses:
Aerophoenix: I'd like to suppose that at least partially, the commonness of the types capable of hurting steels is at least partially in part due to the existence of Steels. Fighting was a terrible type in Gen I, because the most powerful typing was Psychic. In Gen II, Dark and Steel were added, both of which beat Psychic and lose to Fighting. And now Fighting is one of the most lauded types in the game.
nyttyn: Grass is an interesting type, because of course, Ferrothorn is at #3 right now, and loves its grass typing. Grass has representatives all over the place, and a few of them are certainly noteworthy in their own tiers. Ferrothorn needs no explanation. Ferroseed does well itself in LC. Venusaur is terrifying when it gets out in the sun. Tangrowth is a very effective wall down in the lower tiers, Roserade is great in UU, Shaymin-S.... is Shaymin-S. Celebi also fills a variety of potent roles.
Grass isn't a dominating type, but it's not ice, either.
Other responses:
Yes. I think SR is incredibly important to this concept.Stealth Rock: SR in the metagame is a huge factor in what typings are defensively good and bad. Fire, Bug, and Ice are all commonly thought to be awful defensive typing, and all are weak to SR (Flying is at least immune to Spikes). Steel and Fighting are great defensive typings and both resist SR. If CAP 5 can spin or magic bounce rocks away it'll do a lot to help equalize defensive typing even when it isn't on the battlefield. A fast taunt to stop SR in an "anti-lead" role with no SR itself might also be helpful. Conversely if it lays SR or is a spin-blocking ghost type it'll contribute more to the problem
IIRC Slowbro is a good example of this atm, with regards to being able to take an Outrage.Physical Defense: Steels soak Dragon Outrages, and Fighting Close Combats menace Steels. I think a strong physically defensive pokemon which is weak to neither Dragon nor Fighting would both lessen the need for Steel types while not reinforcing Fighting types. Ideally, this pokemon would not be one of the "strong" types itself. Since we don't want to use Steels here, we need either a lot of physical defense indeed, or outside the box solutions like Unaware or Prankster WoW.
Aerophoenix: I'd like to suppose that at least partially, the commonness of the types capable of hurting steels is at least partially in part due to the existence of Steels. Fighting was a terrible type in Gen I, because the most powerful typing was Psychic. In Gen II, Dark and Steel were added, both of which beat Psychic and lose to Fighting. And now Fighting is one of the most lauded types in the game.
nyttyn: Grass is an interesting type, because of course, Ferrothorn is at #3 right now, and loves its grass typing. Grass has representatives all over the place, and a few of them are certainly noteworthy in their own tiers. Ferrothorn needs no explanation. Ferroseed does well itself in LC. Venusaur is terrifying when it gets out in the sun. Tangrowth is a very effective wall down in the lower tiers, Roserade is great in UU, Shaymin-S.... is Shaymin-S. Celebi also fills a variety of potent roles.
Grass isn't a dominating type, but it's not ice, either.