Well the limit in towns is more to do with the performance of pedestrians, so 50kph / 30mph is pretty much the norm throughout the world.
On motorways it is a different matter. The UK limit was introduced in the
1960's and was based on the performance etc. of cars at the time (top speed ~75mph, cross ply tyres, drum brakes, naf steering, solid rear axle & cart spring suspension). These days almost any car can reach 100mph, has suitable radial ply tyres, disc brakes (so can stop from top speed near physical limit), rack and pinion steering and independent suspension.Almost alone the UK has persevered unchanged. In Europe, only Denmark, Sweden and the [motorway free AFIACS] Baltic republics have a lower motorway limit.
So the poster is correct the current UK speed limits WERE introduced for a very good reason, and whilst the limit on shared roads are broadly correct the limits on segregated roads (motorways, grade separated dual carriageways etc.) could do with updating.
Of course if vehicles didn't move at all then they would be almost totally safe.