There is no such thing as a "non replaceable battery"
Replacing the battery on an iPhone is simple - which is a lot of the rest for iFixit's 6s and 7s for iPhone repairability. They aren't glued down, there's a little strip of tape you pull to remove it, and the replacement battery comes with the same strip of tape to hold it in place. Sure, you need a couple "specialty tools" to open it, but you can buy those for next to nothing on eBay, or slightly more than nothing from iFixit.
Even on phones where they are glued down like Samsung, it is just more difficult/messy, but not impossible. If you don't want to do it yourself there are plenty of places you can have it done. Unless you throw away the phone in the trash, it will likely end up getting re-used if you trade it in or give it to a recycler. If they can replace the battery for $40 it is worth it if they can sell the phone for $50 or more.