Blue
Ravenous Bugblatter Beast of Traal
Okay let me break it in half which would look like the following :
+1 Attribute == +1 Skill Proficiency == +1 Tool Proficiency == 2 Weapon Proficiencies == 2 Armor Proficiencies
So why 2 Weapon and/or 2 Armor well frankly you can generally only use 1 at a time someone is not going to wear both Light armor and Medium Armor -- okay Shields are an anomaly when it comes to the Armor category but it is 1 out of 4 so we go with the majority and call it a day. However if you look at the Armor progression it mostly goes like this -- Light + Medium then Shield + Heavy --- and if they halfsies you get Light (0.5) or Light, Medium, Shield (1.5)
Say there was a feat that gave you +10 AC but slow your movement by 10'. People would say that it wasn't balanced against the other feats. For a 10 Dex Wizard or Sorcerer, picking up heavy armor + shield would give you exactly that - 10 points of AC at the cost of 10' of movement. (Hmm, could get 11 AC if you only had an 8 DEX.)
I understand where your generalizations are in levels, but 4 levels of armor proficiency is automatically full + shield for even those who start without, and that's a huge jump in AC. Enough that it wouldn't be okay if you wrote it up as a feat.
In other words, it seems you're saying that each step is reasonable, and I'm replying that the final result is not.