I think it’s good to balance around the top% because that’s where the maximum potential is to be seen and certain things that would be okay in lower ranks might end up gamebreaking in higher ranks. However, the way of the balancing, that’s another thing because there are many variables to look at, which is also what makes it so hard for the developers.
You can balance numbers on a sheet for HP, damage, fire rate, ability cooldown, etc. but you can’t balance player skill.
If you nerf or buff primary fire damage, the impact will be higher where the general accuracy is higher. If you make spread or lock on range bigger, it will likely have little impact in high ranks while being more impactful in lower ranks, while if you decrease spread, the hero will be stronger in more mechanically gifted hands than in others.
If you touch abilities they are a bit less mechanically reliant usually, but for example Echos alternativ fire is way stronger in lower ranks where players often stand still and are easy targets while Echo as a flying hero is also harder to kill for them. Meanwhile in higher ranks Echo is a throw pick more or less in most situations. If you increase the cooldown on a defensive ability like wraith form this will have less impact in higher ranks, as players are more decisive and fights happen more calculated while in low ranks it’s a get out of jail card, so players with less game sense and worse positioning will generally suffer more from this change.
Then take HP, no mechanical skill involved with that right? But then think about the other skillsets in Overwatch, like positioning and awareness. A nerf to HP will be more noticeable in ranks where players are out of position more often, dying faster in situations where they might have been lucky before. On top of that, less HP opens a hero up to new interactions, like for example if you bring them from 250 to 200 HP they can be oneshot by Ashe, Doomfist, Genji blade, etc. which wouldn’t happen before.
Which brings up the next point, a lot of abilities and heroes are balanced around interactions with abilities like damage boosts, meaning the hero itself isn’t allowed to have higher numbers because it would break a threshhold with damage boosts, that would make it overpowered.
Heroes are also balanced around team play components, which are likely less to be found in lower ranks. Taking defenses from a tank makes them more reliant on supports and a coordinated engage. Anas nade is one of these examples too, where in higher ranks the anti heal is super strong while in lower ranks, Ana players usually mainly use the nade as a self healing tool as the other support might not heal them as much or they just take more damage from the way they position.
Now taking all of this into consideration, there is also the fun layer. What can you nerf or change without the hero feeling clunky or removing / adding new combos to the game. Higher cooldowns will make the hero less fluent to play, less combos take away excitement of completing it, etc.
That is the problem I think - Overwatch has too many variables that impact the balancing decisions.
Games like CS:Go or Modern Warfare are more reliant on spread, damage falloff, damage numbers, etc. when it comes to balancing, with everyone being on a more or less even ground. It’s more aiming focus than ability focussed, making balancing easier as you have less to worry about.
Then you have games like League of Legends that are more ability focussed while aim plays a lesser role, where you have to worry less about players mechanical skill when making balancing choices.
Now in Overwatch, you have heroes that require balancing in the style of FPS games while other heroes require balancing in the style of MOBAs while some require both and at the same time you can never just look at the hero alone but have to look at all the different interactions between them too… making it super hard to make balancing changes that won’t affect the game in an unfun way in some form at one end of the spectrum.
