A single mage having some single-target DPS loss doesn’t equate “harder fight” or even “longer fight”. Add-control is important for bosses like Soul Keeper which True Affliction already trivializes. Also just to point it out, it can become a DPS increase against bosses/elites where you would normally have to focus on adds & you are spreading the dots to the boss/elite from the adds(instead of the opposite direction). Arcane Missile also barely takes any time at all to cast, clocking slightly above the cast time of a fireball 2.
Just to clarify I did put in “or working in arcane blasts(missles)”. For single-target situations with adds, it actually can further result in a DPS increase because you can focus less on casting affliction(a low initial damage spell) and instead be casting a fireball or ice lance for example. Then when the timer is about to reset, you simply refresh it with arcane missiles instead of affliction+arcane missiles. Doing this repeatedly only results in additional damage. Also with the triple-rune-type-cast +20% dmg buff, you could do frost+fire+arcane (the arcane getting the 20% dmg buff for aoe) without using affliction(something you’d want to cast before arcane missiles). This would let you easily get more ticks of fire dmg out, buffing your overall dmg and the arcane missile itself would be +20% dmg (not sure if the dot spread is affected by that buff, haven’t tested).
I think True Affliction is already god-tier. I wouldn’t mind playing around with arcane missile refreshing dots, but just thinking about it, it’s pretty clear how strong that would be. It would be a Quality of Life increase, but it also lowers the skill cap.