How does this help? 99% of the population aren't technically minded enough. Most people just buy a wifi router, plug it in (maybe having read the instructions) and that's it. They have neither the skills nor the inclination to update firmware.

The real problem is: assuming that firmware can be updated, how do you run a nationwide update programme overcoming a population that doesn't really care or have the skills to do it.

Vehicle safety standards (mandated annual safety checks like the UK MoT test) is the closest analogy I can think of - in the UK you can't insure your car without a valid MoT. If you were serious, then maybe tying ISP access to updated router firmware would be the way to go.