The point of doing the math is to demonstrate that it does indeed change her damage at ALL energy levels from 0% to 99%, hence, it is a nerf to her overall average damage output.
You do not respawn at 80% energy.
You do not maintain greater than 80% energy in a losing situation.
Energy is harder to build and maintain when playing against better opponents.
Arguing based on fictionalized ideal scenarios completely misses the point because that is not how real games against skilled opponents plays out.
No, I am summarizing posts made by people who demonstrate the same lack of understanding on why changing the curve makes a difference at all energy levels, and not just at the bottom of the energy curve.
As you can see, it is intended as a time saver. 
I can address your argument individually, if you would prefer.
Obviously, 100% energy performs the same before as it does now. That has very little to do with whether it is a large reduction in her average damage output.
Pointing out that the top end of the curve is less of a nerf than the bottom is meaningless because that is not how averages work. Especially since you, like so many others, have claimed that it only matters at the lowest energy states when simple math demonstrates this is not the case.
My examples are of two other sustained-damage heroes that are critically dependent upon their actual damage per second to secure eliminations, because that is how their kits work.
Even without considering breakpoints, it is very relevant, because altering DPS output by 10 (or more) significantly impairs the ability of sustained-damage to overpower healing.
Changing damage per second does not weaken a burst-damage weapon nearly as much.
For example -
- Taking 10dps off McCree’s revolver is a significantly less impactful nerf than taking 10dps off Soldier 76’s rifle.
Another example -
- Adding 10 damage per second to Winston’s gun would be a massive buff.
- Adding 10 damage per second to Sigma’s hyperspheres is no where close to the same magnitude of a change.
It is not a nerf to just “bad Zarya” players.
It does not make her unplayable.
But it is a sizable nerf to her overall effectiveness.
Pretending this is not the case helps no one.
With that said, unless you have some new argument to make, and since so many people appear to be missing the point even when it is demonstrated for them directly, I do not feel as though any further discussion on the subject is productive.