I've flown enough to realize that no one ever thought having a uniform overhead space was a good idea.

I'm currently shopping for a new bag, but my current bag is legal on many airlines but runs into issues every so often from the odd weird plane. I even recall once that it fit perfectly in the bin - it just had to be crammed with great force (and it was not overpacked) to clear the lip of the bin. In fact, it was quite roomy in there once it made it in. Twas an odd day on Delta.

People need to travel with stuff. It's true. Airlines like to pretend they owe people nothing for losing their stuff. Airlines also decided to charge insane fees to check luggage but still not be responsible for your stuff. This created the carry-on problem.

A full flight on a WN can still have empty bins in the middle of winter.

It's the culture and BS of the airlines that has created the problem, not someone putting two items in the bin. Seriously, you boarded last - how do you know what in the bin belongs to who? Did you watch them load it all?