I can be productive for 12-15 hours a day when the conditions are just right, but usually they aren't and after a few hours I need to switch to something less productive than actual coding.
Do employers really see any measurable improvement in volume (let alone quality) of output after 30 hours a week?
I would argue that 90% of employers cannot effectively measure productivity. I have worked in call centers where metrics are tracked - which of course I learned to game - allowing me to work the same hours while making 50% above my peers based upon bonuses.
Knowledge workers are a different type of job with somewhere >50% being fake jobs that exist only because companies have capital and managers and head count means success.
You've got a guy sitting at his desk at 8:55PM on a Saturday night. Impressive. We all know he hasn't been doing anything worthwhile since some time back on Thursday, and you've been wasting electricity on him afterward.
Because in corpo you are always easily replaceable.
So you don't get the refresh, the first 1-2 companies which employs you get the most benefit because you are not drained yet, but companies later in the line got a used battery almost completely drained -- and that's why China also has the 35 yrs old rule.