Why is the garage door chain sprocket cover on the floor?

Something's not right

I took apart the opener to see if I needed to order parts or if I could fix it with local hardware store parts, decided I needed to order parts, put it back together hoping it would still work until the parts came in, and then the sprocket sheared off completely. 

Why is the shaft so skinny by the sprocket?  Oh wait.

I didn't bother tightening the chain last year after already tightening it twice in the last 7 years.  It turns out that after the shaft wears through the bushing, the lip of the plate that holds the bushing wears through the shaft from a 1/2" diameter to almost 5/32".  A Liftmaster 41C4220A kit including the worm gear and bushings for the motor output shaft ended up being $21 on ebay.

So the steel of the plate holding the bushing is harder than the shaft steel?