Jarvy,
I think you will find that only one person has said he carrys too much on board - and that person has good reason.
I know I stick to the limits. I wouldn't have spent so much time producing the spreadsheet that I have, and have about eight different carry bags to fit airline requirements. Plus the fact that I have specialised low weight gear.
In your experience you say that carry on baggage is rarely weighed. When I check in to say "Hi, I'm here to fly with you today" my carry on bag is weighed 99% of the time. Perhaps it is because I tend to use small light weight rucksacks. I do not know.
When I travelled as part of a team (globally) there were always bags going missing, being broken, or being broken in to. You can't just take an airline saying to you "I'm sure you bag will turn up, where can we deliver it to you?". Our response being "some where in the jungle / desert / offshore, but certainly no where near any where". Another one that used to happen was the fact that we travelled a lot, so the lost bag never caught up with you.
In my experience, and obviously others, the 99% that you quote is "tosh".
Your experience is obviously different.
I do believe that most of us are also aware of design limitations of overhead lockers.
I would say that the checked bag system is broken. Look at most of Africa, or even London Heathrow, or Glasgow.