"+1 studded leather"
I prefer the design that keeps magic items out of the balance equations.
The +5 Dexterity happens automatically, because the benefits from investing in Dexterity fighting incentivize it.
Mage Armor = Plate Armor
No, that is false.
Firstly, the characters that can gain Mage Armor without taking a feat will have stats that are either more important than dex, or just as important.
Secondly, even if we imagine the sort of CharOp edge case that the game should never balance around, and assume a character with Mage Armor who got it from their class, but isn't putting anything on the same or higher priority level as dex, they won't have a 20 until level 4 at the earliest, and level 8 is much more likely.
So, what, they're 1 AC above Studded Leather. Scary.
That is the point. When the Str character gains Plate Armor 18 AC, the Dex character would have Mage Armor with an equal 18 AC.
Mage Armor = Plate Armor
Except most characters won't have a 20 Dex until noticeably later. The fact that two characters can both eventually reach the same AC does not mean that they are the same. for the erroneous statement, "mage armor=plate armor", to be true, it would have to provide 18 AC to any character who uses it, without having to dedicate 2 ASIs on top of a feat to get Mage Armor as a ritual. (classes that have mage armor and have ritual casting are going for heavy armor with extreme rarity, so the majority of characters who are actually relevant to a comparison between Mage Armor and Plate are pretty much Fighters. And melee dex fighters can get to that AC level without Mage Armor. Archers too, actually, if they take Defensive instead of Archery, but that's niche.
That confirms the other point. "Mage Armor is a couple extra AC."
In other words, Mage Armor makes Dex fighting strictly better than Str fighting for "half the classes" that lack heavy armor proficiency.
No, it makes it better for classes that lack medium and heavy armor, which it already is, and always will be. So, it doesn't change that dynamic at all. Your argument may as well be that the Dual Wielder feat is broken because that +1 to AC allows a Dex character to get the same AC as plate, thus making dex and dual weilding better for half the classes. Yet, no one I've ever seen in any forum considers that feat too strong, and most consider it not strong enough.
A spell that lasts for 8 hours, is effectively equivalent to ‘always on’. Virtually, the Wizard is swapping out a level 1 spell slot for armor proficiency.
At this point, the design might as well have made Mage Armor a class feature of the Wizard class.
This is the same issue and argument as Hunter's Mark and Hex and Eldritch Blast. I don't care. They didn't make them class features, they made them spells. It is what it is. Making it a ritual does a similar thing, which is fine, and also allows other gish type characters to look and feel right for the player.
In bounded accuracy, a feat that grants +2 attack rolls would be a huge deal. Notice, such a feat doesnt even exist in the Players Handbook.
The game isn't as reserved with AC boosts. Dual Wielder gives +1 AC. There are plenty of other sources of AC boosts. This is a molehill.
Conversely, because of the exact same math, +2 AC is a big deal.
if the designers agreed with you, there would be as few AC boosts as there are attack boosts.
The design math is, roughly:
Light armor +2 AC → 12 AC
Medium armor +2 AC → 14 AC
Heavy armor +2 AC → 16 AC
Meanwhile Plate Armor 18 AC is pretend-balanced by high gp cost.
18 AC is easy to get. Every heavy armor guy with a shield reaches it without plate, and surpasses it with. +1 armor is uncommon. The most common magic item type in the DMG.
5e isn't intended to be as tightly numerically balanced as you seem to think.
Medium armor seems useful enough at low levels, but becomes strictly inferior at high levels, being outclassed by better heavy armor for Strength or light armor for Dexterity. For this reason, I think it is ok to lump medium and heavy armor together into the same feat.
Better to combine medium armor proficiency and medium armor mastery into 1 feat, possibly still a half feat.
The original design goal was to have Light Armor max out at AC 17, and Medium Armor max out at AC 17, and the only way to reach AC 18 was Heavy Armor via plate armor.
When one adds the +2 shield, Heavy Armor comes out to an even AC 20. A number, which the design intended for light and medium to be unable to reach.
Mage Armor can reach this AC 20, if with a shield, but a Wizard rarely uses a shield.
So, the only characters who can get AC 20 with Mage Armor are the guys who are already, at most, 1 feat away from it? Seems pretty fair to me.
