What matters is how good it is right now, not how good it will be if people start using counters because of it. The point is to evaluate a Pokémon's effectiveness in the current metagame rather than the metagame with potential differences and changes - this is the exact reason saying "Pokémon B will be broken if we ban Pokémon A" is not a valid argument against banning Pokémon A.if Dug gets higher in the ranks, more people will use air balloon which is my point. Then you are pretty much forced to attack heatran or magnezone, and tapu-koko still doesnt care about being trapped since it has u-turn. And magnezone can run hidden power ice, since it can easily take out landorus t also.
If Dugtrio is moved up to A and people start running things specifically to counter Dugtrio which decreases it's viability, then it will be moved back down to A-. Not now.
I'm not even sure how you can say with such certainty without a crystal ball that a significant number of people will start running such a terrible item on their Pokémon because Dugtrio now has 'A' instead of 'A-' attached to it in a thread not every player pays attention to.