The dramatic reduction in insects on car windscreens could be "explained away" by technological advances in car design (getting windscreens covered in insects could conceivably have been a problem great enough for someone to have invested time and money into resolving with more efficient aerodynamics of windscreens or whatever). Not saying that is actually the cause, just that many people could rationalise it as such.

The earthworms on a path or road is slightly more troubling though. Not something I'd thought about before, but there is a path through a park where I grew up that used to be covered in earthworms after every rain when I was a child, but the odd times I've been back up since in similar conditions I've not noticed any at all. Being a park it wouldn't have been exposed to more intensive farming or anything else. Same park, just a different time.

Chilling thought that there could be significant and detrimental changes underway in nature, happening on generational timescales so slowly that no-one notices, a little like the proverbial frog in boiling water.