Here’s something I found out while working some button mods today.
The inside of a Sanwa micro switch:
Its made up of five parts all together. The way it works is that on the bottom piece(shown in the last two pictures although a little blurry) has two prongs which correspond to the two terminals and then it also has a small steep plastic ramp with a V cut out of the top. When the assembly is together the ball naturally rests in the V cutout and when you press down on the switch, the spring pushes the ball out of the V and up against the two terminals thus closing the switch.
So now that we know how it works we can guess at why the switch wears out over time. It could be that the ball wears over time changing its shape slightly thus making it sit more naturally in the cutout and causing you to have to press down further to close the switch. Or it could be that the V cutout wears over time causing the ball to either sit more or less on the cutout.
Perhaps if someone has some old microswitches they could take one apart to figure out exactly how they wear.