This is not a good thing. Trying to buy a battery for a user replaceable Android phone is a real lottery.
The problem is, an old battery looks exactly like a new battery and there are thousands of old phones out there.
Consequently, the market for batteries is swamped with companies selling old for new and you just have to look at a few reviews to see I am right. You might get one that's almost new, you might not.
I recently had to change the battery on my mother-in-laws' LG and after a lengthy search, settled on what seemed to be a reliable third party manufacturer. It was ok and cost €25 but I don't see the advantage really over having Apple do it for €49 and in 2 hours (and I've had three changed in under 2 hours).
The phones are waterproof and the batteries are genuine.
If they ever do decide to do this then I think Apple will just cut a deal to replace them at the same price as the average user-replaceable Android battery.
That coupled to security implications-chips on the battery, I don't see them having any real problems with this.