Penn has a $22B endowment, and pulls around 5% out of that annually. That seems to be a reasonably safe number that will give them a good chance of at worst keeping the endowment's size constant. Sure, they can take out more every year (they'd have to take out more than 4x that to match Penn's current budget), but then their endowment would reduce in value every year and eventually run out. That would not be a good outcome.