Defining "dying" in terms of a "research area" makes more sense. This is to be expected over time when progress, or expectations of progress, slows. Essentially becoming a mature framework no longer seeing major developments. This can change if, or when, developments occur that reignites progress in set theory. But presently it's mostly more of a tool set than an invention in progress. In that sense it merely matured rather than dying, and no longer requires the level of research needed to mature it.