If I understand you correctly, "the sky" refers to the greatest amount of damage possible, so you judge everything as a percentage of that.
One issue with that approach, at least from a usability standpoint, is that we don't really know how high the sky actually is, because the game options are not necessarily set in stone prior to making the evaluation. At a given level, you might get a sustained 50 damage per round from a particular configuration of allowed classes, races, feats, and so on. And you might get 30 damage per round, with a more restrictive allowance. But if you actually opened it up to everything out there - DM's Guild, homebrew, conversions, etc - then you could gets hundreds of damage per round, if not thousands.
Without a baseline, we can't tell which options are over-powered, and which are under-powered. Without having a firm baseline, we can't decide which options should be allowed into the game or not, which is one of the primary goals in these discussions. When someone proposes a new class on here, there's no way for us to know whether it's balanced or not, because we don't have a set standard to compare against. Having a fixed standard for everyone on these boards would be incredibly useful.
I'm drawn toward an ensemble of fixed standards, based on pillar builds representing the main strategies, times tiers.
The reason is that some strategies offer advantages that are balanced by reduced damage. A single fixed standard might make a build look weak, whereas against other builds offering those advantages it is strong. Additionally, some strategies change in relative strength depending on tier. A single fixed standard could suffer the same issues as "the sky" in such respects.
A starting point then might be to agree the main strategies. I think some of those are -
Melee one-target* weapon attack, no shield e.g. Reckless+GWM
Ranged one-target attack, e.g. Precision+SS
Melee high-AC, e.g. Trip+Shield+Defense or Shield(spell)+Shield+Defense
Ranged cantrip spam, e.g. Agonizing + Eldritch Blast (+Repelling?)
Offensive buff, e.g. Twinned+Haste or Twinned+Greater Invis.
Unarmed e.g. Mobile+Shadow Arts or <missing feat>+Open Hand
Dual-Wield melee one-target weapon attack e.g. Dual Wielder+TWF+Rapiers (IKR)
*at a time
Once the main strategies were agreed on, a next step could be to agree parameters for various up-times, i.e. how many rounds to average over, how many encounters to spread resources across, number and HP of foes for strategies like cleave and horde breaker. An ensemble like that casts a lot of light, revealing gaps in the design space that players might like to explore, and places where the designer balance estimates have gone over or under. It could require formation of a work group who might argue out the terms, before getting down to creating the estimates.