I've been wanting exactly this for so long. I want to bury an AirTag in my luggage, backpack, etc. and never think about it again. In those scenarios, it doesn't _need_ to be tiny. I'd rather trade compactness for longevity.
However, an AirTag attached to my keys _should_ be small and it's easily accessible so I don't mind swapping the battery as needed.
I fully expect Apple to release the airTag in different form factors. They can then also sell a whole bunch of new accessories. A 'pen' form factor with replaceable AAAA battery might be perfect for myself.
This is really a non-issue. Your phone literally tells you when the tag's battery is low. I'd rather do that once every 365 days than having to carry 2 AA batteries for 365 days/year.
However, an AirTag attached to my keys _should_ be small and it's easily accessible so I don't mind swapping the battery as needed.