The only way they can “fix” Mythic+ is to introduce a cap to how far Keys can go. Until they do that, Blizzard can never balance classes properly for Mythic+ because depending on the dungeons in the season, there will always be a comp that is able to do just a bit more than another comp with their personal defensives + group utilities and cleave potential.
The main thing they need to balance is cleave. Some specs are just…way better than others when it comes to consistency and uptime, but this is the nature of infinitely scaling content. Until they cap Keystone level, this problem is literally impossible to solve.