M+ runs take too long. The average M+ run shouldn't be tuned around a 40-45 minute experience, but rather 25-30 minute one.
I don't want to do anything with other strangers for more than 30 minutes at a time. It also would make the feeling of reward increase with each successive dungeon run because you didn't just feel like you wasted 45 minutes of your time just to go up 1/10 for your weekly Vault "quest".
Another huge upside would be that since the dungeons would be getting shorter, they could get away with smaller dungeons with less art assets in general, and so you could have a higher amount of dungeons in the game. Instead of 8, we could launch with 12 and get more during patches.
Just do it man. M+ blows.