It's not clear if you have to stick with AT&T for 60 days post purchase to get it unlocked or if prior tenure satisfies the requirement. Unless you have a particular need to leave AT&T I'm not sure it matters though whether you buy locked and then unlock or buy unlocked. Here's AT&T's policy:
https://www.att.com/deviceunlock/client/en_US/
What part is unclear to you? On the older style plans, a portion of the monthly charge goes towards paying for the phone. It's embedded in the cost of services so (a) you don't know the actual subsidy amount and (b) you still pay it whether you upgrade or don't.
This is fine if you always upgrade at the earliest opportunity for a full subsidy upgrade. Not everyone does this; some people are happy with the same phone for three years. Or sometimes someone upgrades every two years but the phone goes to another person on the plan as a hand-me-down.
With the newer plans the carriers are beginning to decouple cell/data service from device costs. This is a big win for those who don't always upgrade every line on their account. Not necessarily a big win for those where the older style plans are the better deal. Everyone should examine their own usage patterns and future expectations then model the options to decide which is best for them. As usual there's no single one-size-fits-all choice that's best for everyone.
I will add however that the "deal" on the new AT&T plans is really for family plan subscribers and less so for individuals. I've not looked closely but suspect the best deal for individuals is really to go prepaid and not even bother with a postpaid account until you're doing two or three lines.