You can remove the 9 screws in the back of the controller and fully remove and replace the battery. Going by valves track record they will have replacement parts on ifixit. Also one of the engineers confirmed while speaking with tested that you can easily access the battery compartment and interior of the device and replace it.

Why not all 3 options, sell a controller with a removable, rechargable battery pack & able to play with a cable in while charging, and if you remove it it leaves room for batteries instead.

AA Batteries might not be as convenient to use, but being able to replace them is a great advantage

it could still have a charging port so it doesn't need to be any inconvenient

One of the nice things with Xbox 360 controllers were the rechargeable battery packs. By default, the controllers used 2 AAs, but you could instead use a battery pack. Just remove the AA battery cover, pop in the rechargeable battery, and that's it. You could then connect a cable to the controller to recharge the battery. And, if the battery happened to be empty (or dead because of old age), you could just replace it with 2 AAs, and continue playing. Some of 8bitdo's controllers uses (or used) the same design, but they come with a rechargeable battery pack in the box.

Fuck AAs , not enough power. They can however use any number of user replaceable, user chargeable LiIon batteries.