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.

To the degree that they're actually offended, I think it may be more reflexive that the employee would put an upper bound on the amount of work they would do in a week.

Then why go home or take off days at all?

Because those are a couple of the things that government labor departments do tend to require.

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?

3/5 of vacation time - yes, why not? That's standard for part-time people here in the UK.

We obviously don't have the same healthcare situation as you, but surely it's just part of the negotiation? If 5 days a week is worth a $100k salary and $10k healthcare (don't ask me if that healthcare cost is 10x too high or 10x too low... but that doesn't matter) then 3 days a week is work $60k salary plus $6k healthcare. So offer that person $54k and healthcare. Or $66k and no healthcare. Simple enough, surely?

How about zero vacation time, and I keep the full healthcare? Or yeah, pay me 3/5 the healthcare, that's also reasonable. I'm sure this pice of the puzzle could be negotiated in each case.

> 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.