I did not say that at all, as you know.

I said there will be days when operations on some routes are impacted by a variety of potential factors that push the aircraft closer to its limits. When that happens the airline (and specifically the pilot) needs to know how much weight is on board. This doesn't happen often but there are times when it does.

And once you start weighing people's carry ons then limit enforcement becomes inevitable, if for no other reason than to prevent those claiming to be special from causing problems. Whether those bags are shipped in the cabin or the hold is of course neither here nor there with regard to total load, other than how it impacts balance and overall weight distribution (possibly quite important on a heavily loaded aircraft...), or constraints on total overhead locker space and load limits. If someone is found to be overweight across both of their allowances they will also pay extra, which seems fair and is also part of the contract they agreed to

We are not quite at the point of charging passengers by body weight yet, but I suspect some enterprising low cost is working on a set of 80/90/100kg type body-plus-baggage fares as we speak. In fact some asian airlines are already weighing passengers for load and fuel calculations. In the meantime though it is well documented that the average weight of the walking cargo is materially higher in some parts of the world than others. On a full widebody that extra alone can add up to several tonnes, requiring a similar amount of extra fuel to lift and keep it in the air along a long flight. It all adds up and restricts operational performance and limits.

So it shouldn't really be a surprise that these types of complaints mostly arise on full flights on ultra long haul routes.

Ultimately though it's their plane their rules (which we all agree to and they only need to enforce occasionally). Other airlines may have different rules that they always, occasionally or never enforce. Thankfully the consumer is usually free to choose