M+ is the timer.
By removing the timer, you cause groups to run only tanks past certain key levels since they can mitigate enough damage to live and since you no longer need the DPS to beat a timer anymore you don’t care about bringing DPS roles.
You also incentivize degenerate gameplay like waiting for lust to come off CD before every pull or waiting on the timer to get another B-rez.
Even raid bosses have soft/hard enrages to prevent this.
By taking the timer out you remove what makes M+ M+, if you want dungeons without a timer play regular dungeons, don’t ruin it for those who like it.