All true. However, in the game one's AC determines whether you're going to take damage or not, so it's that threshold we need to look at.
Here I disagree. To put it in 3e terms, you're conflating touch AC with full AC. Sure, Barkskin doesn't make the Druid any harder to touch but it does make the Druid harder to damage...which in most cases, if you're the Druid, is what really matters.
First off, everything has at least two AC values*, based on what the attacker is trying to accomplish. The one used most often is the AC vs. damage, but AC vs. touch or contact is also relevant even if the game system doesn't explicitly say so.
* - which can be the same but are usually different.
The oak tree's AC isn't determining whether you can hit it or not, it's determining whether you'll damage it when you do. 5e doesn't have AC values less than 10 I don't think, but if it did an oak tree's touch AC would certainly be there: it's pretty easy to walk up to an oak tree and hit it with something. But it's not as easy to hit it hard enough to cause it any damage, hence the 17 AC or whatever it has.
So if an oak tree's AC represents the does-it-take-damage threshold rather than the can-you-hit-it (or "touch") threshold, why would you define the Druid's (full) AC any differently?
I'm not sure it's being used to reflect different things at all. The root question, whether you're swinging an axe at a tree or a pillow or someone's head, is this: did you hit it hard enough or well enough to hurt it? The only difference is that an oak tree or a pillow only get their AC from one source, that being the simple hardness of whatever they're made of; where most (but not all) creatures can accumulate AC from two or more sources e.g. skin, armour, shield, dexterity, cover, magic, etc.
Now the whole structural damage vs. hit point damage question of what is achieved on a successful hit is something else again, but that's damage; and we're looking at the to-hit side, before it gets that far.
Lan-"my bigger issue with Druids is this: changing shape should break concentration, but it doesn't"-efan