>> otherwise they carry the risk of the trademark becoming too widely used/generic so it would become meaningless.

if you've seen a picture of an electric guitar in the last 75 years you'd know this horse bolted a while ago. The "classic" styles of stratocaster, telecaster, les paul and SG have been made by everybody since forever. And that's before you even establish if Fender has some form of "trademark" (on a shape!)