They will nope back with DF groups too. The content isn’t for casual players. It requires players move up to the next level of skill to complete successfully. Players with Keys (who don’t want to waste those keys), wont be signing up for a DF version of Mythic+.
It’s a bad idea.
If you want to get into a Mythic+ group, just use the LFG tool to find one.
This is a solution looking for a problem.