He did mention "dual carrigeway" as well in the first line of his OP which, for NSL sections, has the same max speed limit as a Motorway so his point still stands...
It is unlikely any increase in the Motorway speed limit would be applied to dual carriageways too. They are not built to the same standards as Motorways and their suitability for higher speed is more questionable than that of a Motorway. Even the government consultation a few years back which seriously asked the question about 80mph on Motorways did not include dual carriageways in scope.
This is partially already recognised in that certain classes of vehicle have a lower speed limit on an NSL Dual Carriageway than they do on the Motorway.
For me the Motorway/DC speed limit seems appropriate, the biggest issue is the ever growing list of rural 50mph limits. What is the point in these? What would happen at 60 that doesn't at 50?
The most amusing things about them is you crawl along wide, well sighted roads at 50mph passing the odd junction down little more than a blind farm track with a 60mph limit on it. Safety first guys...
If there is a clear and obvious danger that stops 60mph being safe, than frankly 50mph isn't much safer. For clear and obvious areas where a lower limit is appropriate it should be 40mph. Otherwise leave it at 60.
50 limits slow traffic down, frustrate drivers and encourage reckless overtakes from the sort of moron who allows frustration to boil over. They also encourage inattentiveness as you drone along at a speed that clearly feels too slow for the road and they encourage a lack of respect for the importance of speed limits.
Most drivers will respect speed limits that feel appropriate.