First, M+ is NOT LFD/LFR. Simply because the group leader (NOT the game) puts together the group. Whereas LFD/LFR, the GAME puts together the group (group leaders have ZERO input).
So then what prevents players from playing "poorly" so that some other schmuck leaves first? And we're talking the full spectrum from going AFK to just missing a few vital interrupts (i.e. Hideous Cackle in Brackenhide).
Which just means that players who want OUT of the run without getting debuff will be incentivized to wipe twice to THEN be able to leave without any negatives. And again, they don't have to be griefing (i.e. Pulling extra trash) they could just play very very poorly.
Who judges if the player is actually "intentionally" doing that?
Case in point, Rokmora (1st boss in Neltharion's Lair) it's definitely worth killing Skitters on Tyrannical week because Shatter does more damage if there are living skitters. So instead of focusing on these adds, a player could just tunnel boss and occasionally cleave onto the skitters. Heck they could even be top DPS (on the boss) but that's still increases the chances of a party wipe.
Which then mean groups that are pushing keys specifically to increase score (say they want to reach KSH) then they have to purposely wait in the dungeon (which they aren't going to time) until the timer hits zero so they can try again. So now you're going to inconvenience already established groups on top of the issues raised.