I suppose I can say this now without really being biased, although I am still an employee. Plenty of people at !nagios really do care about open source. Nagios does, of course, employ people to work on and support the entirely #GPL Nagios Core, which is completely usable (and as far as performance is concerned, better than the commercial product anyway)

#NRPE is also completely GPL and used by our commercial product and developed and supported by the company. The plugins are open source (licenses on github).

The #NCPA licensed is designed specifically to prevent #icinga and icinga wannabes from using it (op5, naemon...possibly others) and so that's a bit of open washing, but the code is all out there and you can hack on it for your own needs.

I wish someone would extent the olive branch, and there is still some cross discussion in back-channels. It's not very much, but it's there. Maybe, someday someone can bridge the gap.