No longer having the promise of more official content coming for the series has a powerful effect. One of the (many) reasons Touhou stays strong is because generally we get a new game at least once a year, if not more. Even if he took longer now and then, fans know it's going to be coming; we know we're going to keep getting more and more Touhou stuff. But if the series ended (due to death of creator), well... the stuff would probably peak as everyone does their swan song or farewells or what not, and then things would peter out gradually. It wouldn't just -go away-, but, it'd be a lot less big of a thing.
Things that are ongoing catch people's attention and hold it long-term. Thinking about what already happened, what will happen next, eagerly anticipating the future, etc. But then once it's over, all you have is looking back on what happened... it's different. One way to think about it is like how an anime (lol) holds your attention; if you're into it, you think about it a lot as it's ongoing, but once they release the final episode and it's over, within a week or two you're probably thinking about it far less than you used to.