You don't throw away MacBooks, you sell them since they are valuable.

But my time has value too, and upgrading from Intel to the new M series did cut compilation times in half for a particular project I was working on. I'm not willing to sacrifice that much time to just keep on using an old machine. Then there's the problem of reliability. Even if I have a spare MacBook lying around there's a non zero chance my work machine will die at a very inconvenient time (like on site at a customer or when working against a short deadline). After 5 years the bathtub curve starts going up and I don't want to depend on old hardware, even when it's working.

Hand down to parents, children, donate, trade-in or resell.

There's a wide gulf between "singularly dependent on a 5 year old PC" and "just throw it out every year".