You have to manage discretionary PTO. The problem has almost never been people taking too much but taking too little.

At some companies, the directive from the top to management was to make sure people took at least 3-5 weeks of PTO every year. For legal reasons you can't keep official track of this (it will be imputed as accrual) but managers would actively nudge people to take more PTO.

If you proactively manage it, it works pretty well.