A bit like windsock indications being useful in an age where no large aircraft, which are at particular danger to a damaged runway surface, are all doing straight in approaches. AFRU fitted, make it mandatory that a notice is broadcast on the read back, clear threshold markers, including lights indicating the runway is closed, council has these for roadworks, so not talking any increased costs. Yeah, yeah, read NOTAMs etc etc, there's going to be occasions when Joe Blo pilot is slack and bombs into an airfield after unexpected diversion and does not read the black and white. At least being warned before touching down might save some crumpled airplane, like in the case of the metro. The NOTAM defense is one layer of protection, either council forgets to post or pilot forgets to read or is inundated with so many obstacles they miss runway closed etc etc.
It's not that hard and I'm very suprised to learn there was not already signage requirements. Again this is really only an issue at licenced AD, anyone going to unlicenced private strips should be calling the owner prior and doing at least one inspection pass before landing to make sure the owners info is close to spec and that no veloceraptors have evolved on site since last visit. If you watched Jurassic Park 3 you would really know the value of proper inspection prior to landing.