(By "almost mandatory" I mean that at least here, you must use up your vacation days until end of next year.)
Only if you assume that a nation-wide law is the only condition under which non-zero vacation days will emerge.
In California at least, I suspect this has something more to do with accounting than experimentation, i.e. the companies don't have to have on their books the money owed if people were to leave without using all their vacation.
The liability situation is a lose-lose one. You take less vacation because you're comfortable you'll get it and the company gets a liability on the books because you work too hard.
Without the liability, you'll see a lot of people take vacation during layoffs though.
Not necessarily -- quantitatively unlimited vacation doesn't mean that the vacation can be taken without management approval of the specific timing, it just means that its not held to a fixed limit.
The only way for us to come up with a better solution is for people to test things out.
PTO is a bit pathological at my current company, for instance, and if I could change anything about my job, it'd be this. But I've found this is representative of a ton of jobs in the US.
First, basically everybody works when taking PTO. The founders work full-time when they take PTO, but just from home or the beach or whatever. Regular employees, with a few exceptions, don't work that much, but are generally expected, via some unspoken convention, to be available for any emergencies and to reply to emails and bug reports, etc.
You have no separate queue of "sick days". So you definitely don't want to use all your PTO for actual vacation time, because then you're in a pickle if you actually get sick. Or what if you have kids, and your kid unexpectedly gets really sick? You have to use PTO for that. There's nothing quite like using PTO when you're sick and still being generally available for emails and emergencies.
Mandatory vacation time would set a precedent of not being afraid to take true vacations during which you truly don't work.