Apple

Did you even open an iPhone? Frankly speaking, if they didn't have special screws and removed a few of the glued parts it would be more than a reasonable compromise to have to deal with their current connectors to replace a battery. The problem is that even if you have the proper screwdriver you'll have to deal with glued stuff that won't come out easily and sourcing batteries isn't easy.

"User replaceable" can be different from "open a back case with your finger and pop the battery out". I believe if Apple did something like: remove 2 phillips screws from the bottom of the phone and then the back/front comes out (without single-use adhesives) and a battery hold in place by two other screws and one more for the current power conector it would be "user replaceable" enough for most people and situations. This would be simple changes to their current design that wouldn't, most likely, require a change to the thickness of the phone nor a complete internal redesign while delivering a very huge improvement in repairability.