Your desktop doesn't have a battery in it, like laptops do. Batteries are the one component in a laptop that is guaranteed to degrade over the course of a few years and eventually make the product unusable. The battery is glued in on the MacBook Pro, so it will eventually become useless. It's as simple as that, and very unfortunate. It's nice to have the option to upgrade the other parts too, though, and why not expect this, if it's clearly possible, as the Framework laptop shows?