Why should they put their foot down about difficulty, but not about this? I’m ok with them putting their foot down here. Mythic is for committed teams, not for people who want to join a group for one night in the Group Finder.
You’re talking about pugs and farm runs. I don’t think Blizzard needs to encourage Mythic pugs (which will largely just get frustrated and complain) or encourage the style of “I’ll get in on this farm run, then on that farm run, instead of committing to a team for progression” style of raiding that’s become pretty widespread in Heroic.