Pfft Portal?, Counterstrike?, L4D? - these are games that we play while we wait for HL3. Gabe is deluded if he thinks that these franchises are moving forward, or would even be as popular without Half Life. I enjoy all those games, but I don't get excited about them...
Portal2 - Add in celebrity voiceover, 2 player mode, paint and stuff, but the character has not changed at all. What we got was more Portal, but to most people Portal is only interesting because of it's relation to Half Life. I'd say that relationship makes the Portal universe fascinating.
L4D2 - Add in different characters, different enemies, and different levels. It's a good game, but over rated, I'd certainly rather play Killing Floor or Dead Island than L4D.
Counterstrike - I can't fault it, it knows what it has to be, and concentrates on being that - just a fast paced military style shooter with some challenging elements to keep it fresh. But Valve have neglected it from what I can see - it got left off the console versions of Orange Box, personally I'd say it should be pretty awesome as a console shooter, it has that pace and drama that keeps people playing.
Anyhoo, it appears that we'll never get a straight answer from Gabe, maybe it's best to just forget Half Life, and maybe try and conjure up some enthusiasm in 5 years time, when HL3 is released... in the meantime - other companies are filling the heavy-story shooter genre (Mass Effect, Halo, Metro2033 etc etc), maybe it's later than Gabe thinks.