You are correct in theory. However, the present system is so bad that it routinely fails at (eg) streaming Netflix even though this uses less than your adverti

You are correct in theory. However, the present system is so bad that it routinely fails at (eg) streaming Netflix even though this uses less than your advertised bandwidth (by a wide margin). So at present, it would just be a matter of automatic documentation of failures -- and the situation would be much improved if we were up against the limit you propose.