Balancing is actualy very simple…all they need is to watch participation %
Because in WoW there is natural phenomena that when something is OP people swarm like rats to play it, so participation % is best indicator there. Its fairy tale when someone say some spec participation is high because its fun to play, its not fun its OP to play and provide you with undeserved wins, others woud be as well fun to play if they are powerfull like you. This is 100% truth, some people wont just admit it. All specs in WoW are fun to play when they are viable, no exceptions.
So when one spec escalate in participation %, they can just hotfix +5% buff underperforming specs, and this can be done each reset, and guess what they will find sweet spot withing moonth or two…Even AI coud do such a first aid hotfixes for them. So if AI detect there is 5000 discs and 200 resto shamans, it can freely buff shamans 5%, next week next balance round… until participation numbers hits ±20% difference.