Getting the same battery capacity would require a battery much larger than the classic removable ThinkPad battery; you'd need a system where you can remove a battery 60-80% the size of the chassis.
On the flip side, you can still replace the battery after a few years when it loses enough of its capacity; it just requires a bit more work. And for people who want more battery capacity and currently swap batteries for that, the trend towards using USB-C as the universal charging port will make it easier to have compatible external batteries, that will also work with your phone and other devices.
AFAIK, fixed batteries are located in the very same place as the removable ones: at the rear of the laptop. They are not spread everywhere, there is no hyper-advanced design is that respect, and they definitely do not occupy 60-80% of the chassis. They could be pulled away / inserted back from the rear is the design choice was such.
Not typically. The tiny system board lives near the back of the laptop, to connect to the ports and the screen. (Often, the system board doesn't even take up the full width of the back of the laptop, and instead has ribbon cables connecting the ports on one side to the system board.) The battery takes up all the space under the front and middle of the laptop.
> They are not spread everywhere, there is no hyper-advanced design is that respect, and they definitely do not occupy 60-80% of the chassis.
I've seen the insides of many laptops, both in person and via pictures. I've seen battery cells laid out in many different shapes around the system board and other components, including Ls, Us, and Hs, and in multiple packets of cells with minimal connections between them.
As for "advanced design", https://www.apple.com/macbook/design/ made a point of talking about its terraced battery cells to fit the enclosure. And while some laptops might not go that far, I've seen many laptops shape groups of battery cells around other components to make a non-rectangular battery.
And as for volume, I've personally seen the internals of many laptops, and the better the laptop, the higher proportion of the volume that consists of battery cells. I wouldn't have given those estimated numbers if I hadn't seen laptops fitting both ends of that range and various points in the middle. (Some quick searches suggest that much smaller laptops, and lower-end laptops, may dip as low as 40-50% battery cells; medium and large laptops, and those intended as higher-end or with higher expected battery life, have more.)
A new keyboard is like 40-50 € for most models, for example. But don't expect to get a sparkling new system board for 200 €. New rubber feet are like 10 € (but it's the whole set for all rubber feet of the system). For the X20x there are still some parts available new from distributors. Which is like 6-7 years of parts supply.