I was going to suggest cementing a ball-bearing into a hole at the desired position.

That's a possibility....although the ball bearing would be too deep for the standard spring length.

a ball-bearing is round and won't wear out fast.

I came to the same conclusion as to this problem with all of our aging light switches. The plastic on mine was also very worn but not to the extent as yours. I solved mine by putting a small ball bearing behond each of the springs. This had the same effect and also slightly increased the tension exerted on the rockers.

FWIW