Perhaps these companies have such varied requirements that none of the existing TSDB systems fully support? We built our own custom time series db, along with a suite of tools for accessing, slicing, plotting, etc bc (at the time, at least) there was no support for bitpacking data, and storing/calculating certain spatial data operations.

My first job was in 2005 writing a TSDB abstraction over sql server used for BI with defense.

we merged ~30Gb of data per night in about 8hrs on a 32core machine which was severly limited by a platter SAN. We would get about 1 disk failure /6months.

So do u need an actual TSDB? Or are other companies doing what we did?

That was never true AFAIK. I recall one of the core devs did a comparison of storage engines years ago and found Influx unsuitable; that work led to the current-gen storage engine.

That was about remote storage. And that was around the time when Influx itself used pluggable backends. Now influx has its own storage engine, and prometheus too.