I've always speculated that an unspoken reason behind Half-Life 3's continued non-existence is the burden to reinvent the first-person shooter again, just as Valve had done on two previous occasions. Is it enough for Valve to just make a super refined sequel, even if it doesn't have the impact of either of the previous Half-Life games? Well, yes—Portal 2 is exactly that model of follow-up. It wasn't a reinvention, it was a welcome extension of the first game's existing ideas. If the level and narrative design is strong enough, it doesn't feel like diminishing returns.