I am not however very sure the open core model would work that well, people generally dread it as it tends to be crippleware.
Serious question: can anyone name one open core product that is really successfull?
If I were the people behind Nginx I would make my money through selling my technical expertise and support. Pretty much like RedHat make their money.
On the other side, this would make room for some new projects or for older to pick up some new steam (e.g. lighty, cherokee).