I think for many people, most people even, this is not a rational decision. It's not about your time being worth a certain amount of value to the company and 4/5 or 3/5 of availability being worth proportionately less. Some people are offended by the idea of you wanting to structure your weeks differently in the first place & think of it purely as special treatment. They are obsessed with the idea of a five day work week and the traditional employer-employee relationship and accepting employment on non-standard terms would be giving away the employer's implicit power over the employee.

We sort of have this system already in place - it's called part time work, and it's generally reserved for jobs that don't offer benefits like paid vacation time or health insurance.

If I am willing to work 3/5 days a week for 3/5 of the pay, do I also only get 3/5 of the health care coverage and 3/5 of the vacation time of my 5/5 peers?

> Creative workers don't output more than 25 quality hours a week anyway.

This is key. Most employers don't realise what a great deal this is. They are paying much less for probably the same amount of output. The person might even be more productive because they are more rested and can think better.