Trash is the main thing in M+, which is part of the reason I hate M+
Because they are timed events and the combined effective HP of the instance largely consists of trash you can AoE
Bosses in m+ are generally break time. Yea they have mechanics but not as intense as pulling 3 pack at a time w/ coordination on cc, stuns and whatever else you have available. And there are times where you want to pull adds w/ the boss to speed up the run
Mostly because of how multiplication works.
Bosses get neat new mechanics at high keys. Like the parrot boss one shorting you w/o LOSing the shot. Trash can also be kited/stunned and skipped, when necessary. Not the same for bosses.
M+ should be considered as one encounter, not in the raid vernacular of “trash” and “boss”.
How many minutes do you spend fighting bosses compared to fighting trash. (yes even on tyrannical)
Now if you speed up one of them to bring down the overall time which would you rather do big damage on?