I agree with you that carry-on luggage should fit easily into overhead bins or under seats for safety purposes. And I agree that some people try to break those rules by bringing monstrous bags and amounts onto the plane.
But, so many people now have rolling bags that there should be one standard size allowed so the luggage industry will know what to make.
There is one statement, however, I question. You wrote:
"Some airlines impose a weight restriction on hand luggage to keep the costs down."
How? If I can't take my bag into the cabin, I'm going to be forced to check it. So, the weight is still added to the aircraft which means the same amount of fuel. If I check this same bag, numerous airline employees have to touch the bag and that takes time. If I carry it on, I'm the only one who has to think about it. The only thing these bags do is slightly slow down loading and deplaning of passengers. And since most airlines don't have that quick of a turnaroud, it shouldn't