If you look back at, say, Blackrock Depths, there would be a variety of different paths to take when looking for a group. To list them, you had;
- Emperor run
- Emp. run w/princess
- Emp. run w/arena
- Emp. lava skip
- Key Skip
- Arena Farm
- Various Quest runs
- Recipe Farms
Now I'm not saying Blackrock Depths is the bar or staple; there are many negatives about it. But the point I'm making here is that the group finder still does facilitate users to specify which type of run they want to do; that way, everyone in the group is on the same page.
But, Mythic+ disincentivizes designing dungeons this way. They are designed to be cookie-cutter, around the various affixes that could be active at any given time. Where players are forced into clearing trash, skips being disincentivized. Any incentive to include optional bosses, or have a variety of paths becomes lessened knowing that the defined Mythic+ path is all that matters. It ultimately creates more linear dungeons despite having the illusion of choice in what order you want to tackle the challenges.
I feel like there is a way to really make both shine. Where dungeons like 'Return to Karazhan', 'Operation: Mechagon', and 'Tazavesh, the Veiled Market' are the standard, but expanded with a variety of different paths and skips you can take throughout the dungeon. Imagine if these dungeons pulled more inspiration from the free-form non-linear nature of those older dungeons, how much better they could be.