First of all, it's been 3 years since episode 2, compared to over a decade of waiting for DNF. Second, Valve has made other game in the mean time so the wait is justified in that regard. And third: DNF has been in development for over a decade, while nobody ever said they are making episode 3/HL3, they didn't even start so it's not comparable (unless they did start but in secrecy but the point still remains, DNF was infamous because they gave false hope while Valve never said they are releasing it soon).