I don't know why people think hero usage is determined by balance.
It isn't. It is determined by utility, at least until a hero otherwise lacking in utility is overpowered.
Most of the characters not getting picked very often don't need to be re-balanced, they need to be re-tooled such that their kit actually fits how the game is played.
Case in point: Reaper. As a solo flanker whose role is to get up close and personal and blow somebody away his kit is absolutely horrible for how Overwatch is currently played. Has some of the loudest footsteps in the game, has a loud audio callout and long wind-up for his teleport, and is supposed to rely on element of surprise? WTF, Blizzard.
Post-Ana he saw some popularity due mainly to how overpowered Nano-boost was. Was picked solely for his nano-boosted ult which thanks to the speed boost present in nano-boost at the time allowed him to get into the middle of the enemy death ball very, very quickly. Nano-boost gets nerfed, Reaper disappears entirely from the meta.
The flipside of that is Ana absolutely destroys his solo flanker playstyle. Biotic grenade means you're not picking off anybody and sleep dart means you're not making it out alive in any case.
It is actually a really good argument for them to stop adding heroes or at least slow down and think about what the heroes are going to do. The introduction of Ana turned Overwatch into tank-heavy deathball simply due to how biotic grenade works. They nerfed it once, looks like they're going to have to nerf it again.