There are some structural limitations to doing this. Car side impact performance is substantially hampered by the very large holes we need to make the driver and passengers replaceable. Nobody wants to crawl in the top like a tank, and we compromise heavily because of this.
One solution that occurs to me is to look at what 3M has for a thermally stable version of the removable adhesives they use for those wall hooks. How many watt hours of capacity do you lose making space for 1) the adhesive, and 2) lower tolerances to account for human error in centering the new battery? I suspect this could be engineered to be less problematic. Possibly with some sort of tool for aligning the battery and dropping it in (thinking suction cups and some rails to drop it straight in).
In one of my first laptops the battery lid was the battery. But this doesn’t allow for a sleek unibody. Apple sacrifices a substantial amount of functionality for the sake „sleekness“. Another prominent example is the keyboard which they not only kept long after they knew it was bust, but the repair was also made needlessly hard because of the Macbook’s design. There are some laptops where you can replace the keyboard without opening the laptop. I don’t expect Apple to make battery lids and swappable keyboards but the current design is user hostile and environmental unfriendly.
The problem is that a battery compartment has six sides. The lid is, best case, only one of the two largest. Square voids are very difficult to brace around. They started stir welding the braces to the unibody to improve stiffness not too long before they did away with the discrete battery. Then they started manufacturing (assembling, really) their own prismatic cells to substantially reduce the amount of packaging and gaps in the laptops.
The model where this all came together, by the way, got a roughly 2-fold increase in battery life. That is not a small compromise you are talking about. Sure, 30% of that was from OS improvements to manage peak power draw, but the rest was due to better, larger batteries by filling every void in the case with custom, higher density cells (with no shielding of their own).
I do not like that the batteries are bonded to a board that takes $400 to replace. You should absolutely be able to separate those, I just don't agree that it's worth having to carry a spare battery in order to have a spare battery.
One solution that occurs to me is to look at what 3M has for a thermally stable version of the removable adhesives they use for those wall hooks. How many watt hours of capacity do you lose making space for 1) the adhesive, and 2) lower tolerances to account for human error in centering the new battery? I suspect this could be engineered to be less problematic. Possibly with some sort of tool for aligning the battery and dropping it in (thinking suction cups and some rails to drop it straight in).