It would also force browsers to implement multiple slightly different engine modes, vastly complicating the browser code.
There are already a few cases, eg quirks mode vs standards mode and “use strict” mode, which was considered necessary for moving forward, but clearly it also complicates things for browsers. We dont want more modes than what is necessary.
There are so many (in hindsight bad) design choices and implementation accidents that currently exist in perpetuity because of backwards compatibility; the web would really benefit if every now and then we could shed old baggage.