Obviously simple rule... the more you work for a flat rate (salary) the more you lower your pay. Second simple rule... the more you work for a flat rate (salary) the larger the opportunity cost. There are probably better things you could be doing for yourself after 5 - things like oh I don't know... have friends, family, start a business (that potentially does NOT have a salary cap), learn new skills, etc. etc.
I see this attitude pop up all the time where there are groups of folks who feel that everyone that is an employee who works normal hours should be thrown out of the profession. From my perspective, this is an unfortunately toxic attitude in that it degrades everyone as a whole and collectively detracts from our value.