Stop it. Stop trying to make him do it. He won't. He will not do it. He's been very clear with you. Yes, you, seemingly random commenter who's suggested that making Half-Life 3 would be a piece of cake. Former Valve writer Chet Faliszek doesn't reckon it would be. The mythical sequel's also not something he'd be interested in penning, because he's not a fan of having to deal with a tonne of established lore.
As spotted by Peaceful Campephilus Gamer, replying to the comment - which argued the plot of a Half-Life 3 "could go anywhere" - Faliszek made clear that he's got no inside knowledge he wishes to share on whether such a game is in the works. Instead, he used it as jumping off point to outline the major hurdle that would put him off working on a game of its nature. Namely, that the plot, in some players eyes' wouldn't be able to go anywhere because of the need to fit with events and circumstances established in the first two games.
"When people ask me 'Oh man, don't you wish you could?' No," the writer said. "No. I almost never want to touch something that already has some kind of lore, some kind of back history about it. And I don't even like Left 4 Dead or anything. I don't want to touch anything old. I don't have to have people who remember stuff better than I do yelling at me about changing some history of lore from 50 years ago at this point or some set of rules."
Faliszek cited this as having been a sticking point when talking to Bungie about working on one of their games. "[Their games] have so much lore," he continued. "They have so much lore and I'm like that lore terrifies me. I have no idea. I don't know that much lore about my own life, let alone your games. I don't want to have to write inside of that."
So, he's not interested in writing any sequels, as Half-Life 3 would be, referring to any such game as "a disaster nightmare that I never want to do". Seriously. He wouldn't want to touch such a game with a ten-foot pole, or even a grav gun separating [him] from that ten-foot pole".
While some would inevitably see it as a fun extra challenge to try and carve out a memorable new story within the confines of an established universe, as we see done all the time in games, you can understand why writers like Faliszek - given the option - would choose to forge a path unencumbered by the need to know previous or source material like the back of your hand. I'm not enough of a Half-Life head to know how big a lore knot a third entry might have to untangle, but two games is plenty for the maze of narrative threads and loose ends to weave into a menacing shape. There's also the weight of living up to an established reputation in the case of beloved or iconic series like Half-Life, which is a burden as much as a blessing in terms of knowing an audience already exists for what you're creating.
Ow! Ok Chet, I already told them, you're not interested, stop jabbing me with that pole!