High ground for archers is only an advantage because it makes it difficult for them to be attacked in melee, that's it. All archers need is "line of sight" to a target, higher or lower doesn't make the archer a "better shot" and as stated by the OP (fantastic breakdown BTW) it is in fact more difficult to hit something "top down" because visable cross section of the target decreases the higher you are relative to the target.
If archer A) is on high ground and archer B) is on low ground and both archers have zero cover then the archer with the high dex will have the advantage. If the archer on high ground is behind ramparts then his/her "line of sight" is just as restricted and "chance to hit" remains the same for both archers, both have disadvantages on hit. If I cannot see you then you cannot see me.
I think people think of company fire for this advantage. If you have 200 archers behind a wall firing blindly over the wall being directed by a crow then chances are one of those 200 arrows will hit someone on the other side. If you put one archer on either side of a wall and they blindly fired at each other regardless of height they wont hit anything.
The only exeption is crossbows. You can remain in cover with a crossbow and aim and fire. You don't need to stand, draw, aim and fire like with bows. You cannot fire a bow sat down hiding behind a box BUT it takes far longer to crank and load crossbows. But then you only gain defence/stealth bonuses.
This isn't about 5e rule puritanicalism, it is again stated by the OP "advantages" come from class and come with "disadvantages". Giving all classes the same cheese mechanic is simply not D&D.