I've seen people carry some ridiculous amounts of carry-on onboard and I truly wish CX would enforce their own policies a bit more. It is not fair that I don't have space for my backpack because someone decides to bring 15kg onto the plane (14kg of which they probably won't even need at all throughout the flight), or in my case last year, 2 people who brought multiple sport equipment bags that took up 2.5 overhead bins.
Sounds like your argument is really about bulk, not weight. My 15kg was in a standard roller carryon, which would fit in a standard baggage sizer at the gate. I've seen a lot of backpacks in my time that would clearly not fit in a gate bag sizer. So the question becomes "should stepped-up gate enforcement really be focused on volume?" Within reason, weight is not going to create issues with other passengers not having enough space for their carry-ons.