[kick]

do you think there's a threshold of being 'healthy enough' that online communities should reach?

They should obviously aim to be as healthy/good as they can be. "Healthy enough" is just a baseline.

is this threshold constant?

Nothing is constant when the participants aren't constant.

can it be met then lost then met again?

Of course. A community can get worse and then recover, though drastic recovery is very rare.

I also find your suggestion of examining places that were 'fantastic for year but died to events' unrelated to the core function of the community really intriguing as well.

Glad to hear!