[mtsmithhn]

When your chain stretches beyond a certain point and does not get replaced it will begin to wear down your gear teeth due to excess friction against the sides. Eventually your gears can become worn enough that they will cause skipping (aka chain slipping off the teeth).

[kazinator]

Yes! Stretched chains (not actually stretched, but elongated due to link pin wear) will devour gears.

I have experience with this. Changed a freewheel on a bike, but not the chain, which was stretched. 11 months (!) later, the freewheel was garbage.