Hi! Thanks for the rate Eggbert!
I'm really tempted to agree with you on Heatran; it would really help out against Sun teams and Volcarona in general. But there is a problem, and it's the same problem I have with Terrakion over Latios; an increased vulnerability to fighting types. Heatran is not a physical wall, which Skarmory is. This is critical to keeping dangerous physical attackers like Terrakion, Dragonite, Garchomp, Breloom, etc. at bay. Heatran struggles against fighting and ground attacks, which all of the above can carry. The Air Balloon is only helpful for one switch in before it gets popped by an attack. Once that happens Heatran is totally vulnerable to all sorts of attacks.
Well, bottom line is that without Skarmory, the team struggles much more against these very common threats, where as with it we struggle with primarily one pokemon, and we have ways to work around it. I think I would rather have Skarmory on this team than Heatran, sorry.