Well it's good to hear that it's done properly else where! I absolutely agree with the salary difference. I'm early enough in my career (no spouse or children either) that I'm willing to spend more time for more compensation, but I can see myself valuing that extra day off way more in the future as well.

I would expect to see more than a 20% salary drop, sine the cost of your healthcare and other benefits doesn't decrease when you work one less day.

What if productivity stays on a similar level?

To add, that has been the results of studies like this.

Honestly I think I may be even slightly more productive, but whatever, I make more than I can spend, I don't think any amount of money would make me work 5 days/week (I mean that quite literally, unless it was maybe for a couple months, and in the 1e6 ballpark, my time on earth is just too valuable to me).

Hi welcome to Europe.

I'm guessing this comment is along the lines of "companies don't pay for healthcare in Europe, so this doesn't apply", but there are other sources of overhead. Anything that is not a profit center is a cost center; buying you your workstation and software, having HR around to resolve disputes between employees, etc. Those things still happen in Europe.

I tried multiple times at multiple companies to negotiate an additional month off per year in exchange for a 15% decrease in compensation. I pointed out that giving me 8% of a year off for a 15% decrease in compensation was a pretty good deal. Nobody ever bit, unfortunately. Not even any counters.