I largely agree with you. I used to work at Cochlear (hearing aid implants) - their "headphone" equivalents actually had neatly replaceable batteries 10 years ago, it's doable! Larger than airpods, to be sure, however i'm sure Apple could shrink them nicely. But... would anyone want the ability to replace batteries? I wouldn't know. I suspect most people misplace them before the batteries go bad.
Batteries go bad in like a year or two. They aren't so easy to lose as you'd assume since they go right into the case after you take them out of your ears due to the relatively short battery life.