Do they lose their trademark if it gets used generically, or if they don't try to prevent that? Because common sense would imply the former, but then this video would just be a big admission that they already lost, no?
"Everywhere you go, you see this scratchy, hairy fastener and you say 'Hey, that's velcro!'"
If 3M comes out with their own hook-and-loop fastener and calls it "3M Velcro", the court would say "you can't sue 3M over this since you clearly don't care when people say Velcro". This whole video is just to establish precedent.
The idea is that you can rely on the name/packaging/etc. to actually indicate that the product was made by who you think it was made by.
If a trademark becomes generic enough, then it stops serving that purpose and so is no longer able to serve its protective function.