> Top universities already have plenty of brand recognition and get way more applicants than what they can accept. The monetary value of any additional marketing is probably very close to zero for them.

You can apply that reasoning to any large organization: once it's known/famous for its excellence, an overwhelming majority of their marketing investments have small return. Still, they continue to invest in marketing because they know that this excellence has to be maintained. That's exactly what they should use MOOCs for - and they initially did