"Touhou is dying" is a combination of favoring the "golden days" of when you first got into the fanbase, the probability that you joined in the circa-2007 explosion of new fans and content, some fans moving onto other fanbases and other things just getting popular, a silly perception that newer official works are worse, and a personal waning interest in the series, among other things. It's such an unbelievably silly notion that it's become somewhat of a running joke, although I don't think that guy was joking.