Earlier quoted context omitted.
But that would go against > I believe they also said they don't want to do more of the same I've heard this too, with Half Life they want each game to be groundbreaking. Arkane have done great games, but they're not all groundbreaking, and arguably none of them are to the level that HL1/2 were.
That's why they should hire someone else to do the leg work (That's what they used to do anyway). There isn't exactly a lot of unique things in Portal 2 compared to the original but the sheer polish and testing Valve applied to their games (when they want to) is still unmatched.
But Gabe has said Half-Life specifically is what they consider their ground breaking series now. So there won't be a 3 until they can push a boundary again like the first 2 did.