It always annoys me to hear people saying: you should move to another engine. Being told "you should accept that obsolescence" makes me feel like I'm consulting a psychologist. I don't need one (at least when it comes to my modding preferences, I guess I could use one for my odd exhibitionist disorder).
We know the age of the Goldsource engine and we're not sticking with it just because we're deliberately ignoring the reality unlike what you're saying. There are good reasons to stick with it, a good part has to do with personal preferences and also passion for the original game. But the Goldsource engine still remains relevant today for other reasons. It's got a distinct gameplay and graphical atmosphere, there is very extensive knowledge of the engine which has been accumulated and documented over the past 20 years, its modding experience is time-tested, and last and far from the least: it's got a huge legacy of content that would be unusable from another engine.
I'm not a Goldsource die-hard fan, but I don't think you should see people who stick and want to do more with the engine as closed-minded people who refuse to accept the reality.
I don't think laziness is a word that describes more than 10% of the Goldsource modders (<- percentage that is completely arbitrary, trust me!)
Unity is closed-source, I believe.