Bit of a tough question, that. Metrics is pretty much the best indicator of performance when it comes to these things. Going by vibe isn't really the most reliable.
I guess if it comes down to it, you could say an increased volume of engagement on your posts (for example) is a good indicator beyond the usual metics. Repeat customers, perhaps brand recognition, the type of followers or audience you attract, those are good ones too.
That sort of indicates good longevity before it reflects on the metrics or stats.