This is concerning to me, not because I use InfluxDB, but because my company is also a "pure play" OSS vendor. So it's always concerning to see a company fail to achieve success with the pure-play model, and wind up having to go "open core".

Also, there were some tentative thoughts about using InfluxDB as the main long-term storage backend for Prometheus, but that has become pretty much uninteresting now that clustering support (needed for LTS and durability) is basically cancelled for the open-source version.

True, but consider this: Given that InfluxDB (core) is Open Source, there's nothing to stop somebody else from coming along and building a version with clustering support. Whether that will happen or not is, of course, an open question.