Just another data point. I still use my AP1, get about 90+ minutes on them for phone calls,, AP2, still get about 2 hours on them, and APP, still get about 2 hours on them. all for calls, and pretty much all of them were used ~4 hours per day during M-F, sometimes up to 8 hours per day. For the AP1, introduced in Dec 2016, that's not bad. for the AP2 introduced a year later I think, so nearly two years of use that is pretty great.
But yes, they are really not serviceable at all I can't imagine being able to have a third party replace the batteries. The teardowns that ifixit dig looked pretty destructive overall. It's probably POSSIBLE, but not realistic.