Again, the point is to mechanically incentives a more modern feel including eliminating armor as standard. That means making armor less effective and compensating with some other defense (probably proficiency). Of course there might be specific circumstances under which the solution to this particular adventure challenge is armor.
I'm not sure 'eliminating armor' was a design goal, so much as one particular response to a world where firearms are common. And, that response is predicated on the common firearms effectively ignoring armor.
That response makes sense in a purely near-modern setting where the primary antagonists are almost always wielding firearms.
The thing is..that kinda isn't the case for monsters in D&D. Even if you replaced every weapon in the monster manual with some sort of gun, you'd still be left with an enormous host of teeth, claws, tails, and pseudopods, not to mention psychic screams, acid spit, and disintegration beams.
Further, dungeons mostly aren't configured like pre-modern battlefields with enormous open fields and unbroken line of sight. And adventuring parties are not typically set up to act like armies when clearing a dungeon. You don't have the adventuring party on one side and all the dungeon inhabitants on the other and then let them duke it out. Instead, they function more like special ops units, avoiding fights where possible, and only taking fights that are as unfair as they can make them.
As a result, I would expect 'adventurer' loadouts to be idiosyncratic when compared to the typical soldiers in the setting.
Because the threats they face are idiosyncratic within the setting.
