It’s not 1dimensional.
Yes supports get countered by anti, but if your teammate refuse to take cover, got anti naded while overextending etc, it’s his fault and not the fault of Ana.
You could say the same about Window or MAashe, they counter every squishie, but their is something you can actually do against it, same goes for Ana. OW is a team game.
There are many ways to remove people from a fight.
You know how dangerous discord orb for a maintank is?
A pro player will use Zaryas Bubble to cleanse it, even if it mean she get no charge for it.
If we start to nerf or remove anything that is actually powerful and influence teamfight dynamic OW would be dead.
And about how often it is actually used offensive: