These guys have given the world a lot and they deserve to make some money. Without people like them we'd be running abominations like MS IIS.
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).
canvas[1], the LMS that a lot of of universities have been adopting is open-core and has been quite successful over the past few years.