My main question is why observability data needs (or benefits from) a tailor-made database instead of a general purpose one. In 2025, anyone working on observability who told me they have to build their own database, I would be very suspicious!
It may not be named "database" but actually take the place of a database.
Observability vendors will try to store logs with ElasticSearch and later find it over expensive and has weak support for archiving cold data. Data Warehouse solution requires a complex ETL pipeline and can be awkward when handling log data (semi-structured data).
That said, if you're building an observability solution for a single company, I'd totally agree to start with single node PG with backup, and only consider other solution when data and query workload grow.