> there are not many companies hiring you for full time at $x that will allow you to work 3 days a week for 50% of $x, much more likely they will let you go and hire somebody else (unless you've been there forever and are the only person that can do something, in which case the company should worry about you getting hit by a bus)
Your calculus is a bit off. A company will not hire you part time because employees need time to learn the job. This includes the culture and political aspects.
Once you've been there for awhile, that all changes. Now you're a part of the company, so the company is going to be way more willing to jump through hoops to keep you around. The reason they wouldn't let you come in at 3 days a week, is the same reason they will start letting you (if you negotiate it right) after you've been there a year or so. It's tougher and more expensive to assimilate a new employee than it is to make a productive current employee happy.
If you can show to the company that the bottom line won't get impacted by a proposed change, make it clear that you'll leave unless you get your way, (hard to do convincingly) and offer an acceptable path to move from A to B, then you too can find work/life balance.
It's not impossible, but it's not easy either. I'd put it as being slightly harder than getting a 20% raise, which I've done. Now that I think about it, if you want a 20% raise, then one path to getting it might be to start the process of negotiating something like this, (to let them know you're serious about not being happy) and then come in one day and ask for more money. If you play your cards right, they'd be much happier to open up the checkbook than to deal with the prospect of having half their workforce ask for the same thing you just got. (that's the real reason they don't want part-timers. Even if they can trust you not to abuse it, what about the next three people that ask?)