Is there any other game that is as "mod friendly" as Half-Life 1 & 2?
Don't think so. Lets be honest, now days people only create GoldSrc mods because they do not require much effort. VHE is an AMAZING tool that is easy too learn and you can make awesome mod only with it, add some new models, sounds, textures and you will get entirely new game. Not sure if there is any other awesome game like this. Half-Life 2 is 12 years old now, but engine is so good, that if you make a mod today it will not look outdated. CS:GO is made on modified Source engine and it is insanely popular. There are still people who play and make original Doom mods, so I don't see why this community could die. Unless there could be another developers hub somewhere.