I guess I'm thinking about this from the perspective of wanting heavy armor training to be reasonably accessible, to give more characters the option of prioritizing strength over dexterity. Spellcasters are probably the more common use case, though.
As I think more about this, maybe it's actually the higher levels of armor training that should be easier to get, since light armor is a +2 AC boost to anyone, while medium and heavy armor combined only add 1 AC over what a dexterity-based attacker can get "for free". I do wonder if it's really necessary for heavier armors to have both strength requirements and additional training requirements. Perhaps these could be re-written as alternatives. Plate armor for instance, might have a requirement of "Heavy Armor Training OR any Armor Training with 17 or higher strength".
I think it is a mistake to think of heavy armor is only 1 more AC than light armor.
Yes, it starts like this and ends like this. But there are quite a few levels where you are 2 or even 3 points ahead. At level 5 or 6 you can probably afford full plate. If you and the dex character wants a feat, this puts you 3 AC ahead. If the dex character starts out with 17 dex, which is possible, you are only 2 points ahead, but that leaves the dex character with only a 15 top in their secondary stat.