Even the new iPhone 7 (picking it as Apple is potentially the worst at user friendliness re: batteries) can have its battery replaced fairly easily if its aging. Yes, it's not as simple as popping a latch and snapping in a new one, but it's still very achievable and for a reasonable price (unless you go to Apple that is).
Having a replaceable battery has easily extended the life of my Samsung Galaxy S3, which has lost the ability to charge through the micro USB port (from what I understand this is a fairly common problem).
There are certainly other ways to fix this issue (ranging from switching to inductive charging to, if your good with soldering, actually swapping out the micro USB port on the board). But swapping out a battery is so simple, even not very technically inclined people can do it.
I'm not sure I could say the same for something like a iPhone 7 battery swap (such as this -- https://www.ifixit.com/Guide/iPhone+7+Battery+Replacement/67... ) -- I agree it looks doable, but non-technically inclined people might run away from those type of instructions.
There are numerous places across the globe that will do the work cheaply, usually for about the price you'd pay for an extra battery for the few modern phones that allow you to swap batteries still.
