I feel that airlines being this restrictive works for a variety of passengers, but for others, when selecting our beloved national carrier, the limit is too restrictive. There is a general assumption that because a person has over the carry-on limit they are trying to do something nefarious, but as others have pointed out, you can have zero clothes in your carry-on and still have the majority of items in the hold, but still need to carry more than 7kg on-board.
Certain items don't travel well in a case that goes in the hold (not even with a hard case and padding) such as laptops, hard drives, dSLR cameras and lenses, etc.. and sometimes just two of those items can exceed 7kg, so just because someone has the need or requirement to travel with any combination of the aforementioned gear, that doesn't mean that they should then be denied of any reading material or items that help make their flight a comfortable one.
So people with no intention to be "troublesome" can have genuine needs, yet the airlines won't even allow them in most cases to PAY extra even just to cater for their needs. I'm not suggesting people are then allowed to pack 30kg of carry-on luggage, but saying there are safety risks with >7kg to me points to a flaw in the airline's planes safety measures, especially when other airlines can safely deal with greater weights by comparison.
What I find even crazier is that a person can pay them to carry over 100kg extra of luggage if they purchase excess luggage in the hold, yet they can't even purchase 3kg extra for their carry-on.
I understand the need for sized based restrictions as per BA, Easyjet, etc, there is only so much physical space, but this is not the criteria that they are going by here.