I think that there's still a big contingent of players who played HL2. It was released on basically every system and the worldwide gamer base is arguably larger now than it was back in 2004.
Also I don't think this can be compared to Duke Nukem Forever. By the time that game came out, I don't think anyone was realistically expecting the game to be worth a 15 year wait (Christ, that game was in development half of my life), because most educated people realized that that wasn't delayed so long so they could fine tune it, but because the project was headed by a complete control-freak/perfectionist. Hell I remember reading stories about how it was a big deal that DNF switching game engines after the
first big trailer had been released. I can't imagine how awful it must have been for those developers, knowing how much of their work had to be scrapped, and constantly stuff scrapped and redone again and again, not because what they were doing was wrong, but because they had an insane boss. I would easily pay $100 for a tell-all book from the people who worked on DNF.
Half Life 3 on the other hand is being handled by Valve, who have a pretty damn good reputation behind them. Honestly I think the delay isn't so much to be perfectionists, but because Valve is developing something very special and unique. HL2 introduced the Source Engine, which is still being used today (although with Unreal out there I'm not sure why.) And Valve has proven with games like Team Fortress 2, they don't mind changing things up to do something different.
Still, like I said I'm not waited with baited breath for Gabe to come out and formally announce HL3, but I think that as long as they do it before 2020 and make it half way (no pun intended) decent then it'll be considered a success.