To be honest mate up until a couple of years ago I was asking myself the same question. I loved bombing it down the motorway as fast as I could get away with. I did brake tests in my RX8 which had huge brakes, and being low and light with big wheels it could stop on a dime, and so obviously the limit is outdated, and it is.
Now though, I don't see the future of motoring to be people bombing it as fast as they can down the motorways because you can't do anything but hope to get to your destination asap. Some will tell me it's several decades away, but I don't think so, I think as cars become electric and self driven (and let's face it, self driving on the motorway is the easiest for any autonomous car), we'll be driven by the car and while self driving cars now still require full driver attention or they tell you off, in the not so distant future, I think laws are more likely to change to allow for autonomy than to increase speed limits, and most will be content in that they can use the time to read, write an email, browse reddit, or whatever it is they normally do with their phones. There's law that's about to change regarding the use of mobile phones while driving, and that's to allow us to use our phones to park our cars, like the S and the 7 Series can do via the app, so it's already in motion.