What's your threshold for packing "too much"? Do you believe that everybody should be able to get by with an under-seat bag? Or are you pretending that the overhead bins are large enough for everyone to bring a standard piece of carry-on luggage?

Either way, it's still possible to check smaller pieces of luggage, and if most passengers could do that economically then loading and unloading the plane would be a lot quicker, saving the airlines non-trivial costs.