I think the same. I run mythic dungeons on my characters which already overgear the regular loot drops from mythic0 because sometimes I get a quest from my OH missions. Yesterday, the healer surprised us with a mythic+ 2 Warden's Vault run and we almost made it in time (one of the wipes which had cost us to lose time was on the healer who did not know how to handle some mechanics of the endboss fight... ).
But besides that, the only benefit of premade groups for me is that I can select DPS which I like to have (like, take a brez or BL class if we don't have one already), because I can only do random groups. In the whole time, I only did 2 mythic dungeons with our guild tank, and 1 more dungeon with guild tank and healer. My guild is very casual, but also RL friends, so I will not go for another guild, and have to stick to random groups (also, I cannot commit to schedules due to my job).
They only need to add a "random group buff" like we had in random heroics some time ago (and probably still have, I don't know), and set an entry barrier which would be higher than with premade groups (higher item level, requirement to have finished heroic version of the dungeon).
Edit: mythic+ should be premade.