While I agree that there are better items than Sunfire Cape, and there could be better itemization for Giant's Belts in general, I have one primary issue with the OP's argument:
Originally Posted by DivineBovine
For the components of Chain Vest and Giants belt, 1800g investment right there, plus a 800g combine cost, you are getting:
0 Extra Armor
20 Extra Health
Unique: Deals 40 magic damage per second to nearby enemies (400 range) AKA the passive.
So essentially you are paying 2610g for 40 magic damage per second in an aoe.
I'm sorry, what?
You just said there was an 1800g investment in Giant's Belt and Chain Vest, then discounted that to a gross oversimplification just to poorly help your argument. You're paying ~800g for the burn passive.
However, in support of your argument, I don't believe the passive is worthwhile on Dominion. In SR, it helps champions that have difficulty farming, farm. It helps them maintain presence in teamfights.
In Dominion, I don't believe it passively interrupts caps (though if you're standing that close, you can just auto-attack). It doesn't do much damage once you factor in magic resistance. Teamfights are smaller; largely relegated to small skirmishes. It doesn't help with dueling. Finally, you're simply not focused on farming minions unless you're bot lane.
The item shouldn't be 'removed', per se. Another armor + Giant's Belt item should be created. However, we have to keep in mind the implications good
Giant's Belt itemization has on the health of Dominion.