It's replacable, either DIY if you tinker for <5% of the cost of the phone, or under warranty for ~10% of the cost of the phone. It's typically done either 0 or 1 time over the lifespan of a phone. I can't really see how that's a big problem. I haven't owned an iPhone yet that hasn't done 2 screen replacements for every battery replacement - and changing battery when you are changing screen is even cheaper.

I wouldn't want to have a user replacable battery behind a hatch if it made the phone worse in any way (less water sealed, less sturdy etc).