Right. We’ve been trying to educate community builders to step away from vanity metrics for a while now because they’re not necessarily indicative of health. That said, every community needs engagement of some type in order to achieve its goal/s, whatever they are. Fluctuations might also indicate an issue that needs to be addressed so they’re important to monitor at a high level.
But engagement stats aren’t comparable across all communities. You need to take into account community type. A support community for a popular product will look very different to a niche community of practice for instance. Both might be ‘healthy’ but those numbers will vary greatly.
I’m not sure there is a one size fits all metric. For a CoP I’d look for a high DAU/MAU score to indicate health, but a similarly high score for a product support community would be dire.