I don't think it really matters how soon after the original it comes out. The two variables are quality, workforce and time. If you can keep quality at least consistant with the original then it makes sense that when the workforce is increased (let's face it, valve is a company that many devs would want to join) the time it takes to make would decrease. Waiting another year just to get a better reaction seems silly to me, because both consumers and developers are worse-off.
TL;DR: Just because valve is getting faster doesn't mean they're putting in less effort. It just means they are working harder or have more workers - either way we're getting good games quickly and they're getting paid.
