1) What is the "good reason" call-in tie to tiers was removed?
It greatly limited possible BOs and therefore led to a stale, boring, predictable meta.
2) "Skipping tiers and filling the gaps with doctrinal units" is though exactly what I and some others think is part of the problem in CoH2. Its cutting out tiers, and tier units. Its also cutting out non-Call-in Commanders with an otherwise interesting array of abilities too. Its reducing diversity, and making the game call-in centric and dependant.
That is the design of some doctrines. And you can't see the real problems, which are:
1)Doctrinal units are just raw upgrades over stock ones
2)Costs often prohibit using normal tier armor with conjunction and lack of doctrinal unit hard caps, encouraging spam(mainly for super heavies but this doesn't exclude medium armor)
3)Average at best cost effectiveness and performance of stock units, especially for soviet side as OKW and USF doesn't have this problem.
3) You say my "simplest solution" is not best. What do recommend you instead?
Something that fixes actual problems, not a half arsed solution that doesn't make sense.