I think it depends on the group. I’ve been in several that have lasted multiple years. They tend to be focused on one specific hobby or demographic niche and then offer a wide variety of activities within that niche. Additionally, have multiple organizers and a large enough funnel of new joiners that it always feels big and welcome and established. I have no idea how they got to the stable position they’re in now, but these meetups are more like self-directed communities than they are going-somewhere-for-one-thing interest groups.