This author presumably understands but buries the lede that for an endowment of $15 billion a university would typically only spend 5%, or $750 million, annually.
So "a mere $400m" is over half of the annual funds from the endowment (not including tuition income and donations) that might be available to a university with such an endowment.
It should be relatively obvious that spending into the principal of an endowment is not a sustainable practice over the long-term for universities that are operating at the scale of centuries.
At a scale of centuries, monetary units themselves become highly unstable. On this time scale it was recently that people thought deflation was good for the economy, and that spending 5% every year would be reckless.
It should be relatively obvious that spending into the principal of an endowment is not a sustainable practice over the long-term for universities that are operating at the scale of centuries.