Brutalizer, Youmoo, SotD (which is rarely seen outside of Jax games), Last Whisper, Stark's Fervor and Black Cleaver
Abyssal, Malady (mostly used on magic based DPS), Sorcerer's Boots, Haunting Guise, and Void Staff.
That's 6 to 5, not 6 to 3.
Second, MR items are more expensive, and it's harder to get high MR than Armor in 3 items unless you stack Force of Nature (only recommended on Mundo or Cho'gath).
Third, while Sorcs cost the second most of all boot types, they are the only boots in League that increase single strike damage output while DPS don't have an ArPen boot.
Last, there will be balance problems in Dominion. No doubt about it. It's called "they just released a new map that has several different new items, buffs, and objectives that's available to all 82 champions" I mean they probably tried, but it's going to take a couple months before Dominion is close to any form of balance other than off-balance.