Just thought about the last 4 years of raiding as a mage. Correct me if I am wrong, trying to remember when I had to play which spec.
T7 starting Fire (FFB), Arcane pushed ahead somewhere in the content
T9 Arcane, then Fire got buffed because it lacked scaling but still was behind fire mostly
T10 Starting Arcane until T10H Gear go Fire ahead due to scaling
T11 Fire, then Arcane pushed ahead again
T12 Arcane, then Fire again
T14 Fire until now