And batteries don't last forever. When you upgrade to a new phone after a few years you'd likely need a new one anyways.
Worst case scenario just sell the old one on eBay if it's still holding a good charge!
Selling a thin phone with half a battery where you have to buy the other half and keep it attached to get a proper battery runtime (turning it into a normal-sized phone) can't be the solution Apple intended. At least I hope so. And that battery doesn't fit other iPhones as the camera bump of those other phones is in the way.
I don't really understand all the complaining since it's merely a variant of the iPhone for people who prioritize thinness over battery.
For over a decade, HNers have complained that they don't want thinness to be forced on them and that there should be a separate SKU for it. Yet when it finally happens, HNers complain about the trade-off.
3rd party versions of course, the official one was much more expensive than that.