[gk1]

The moment you cross 10M items or 100 QPS then scaling such a system becomes non-trivial. That's not a high threshold for any enterprise software company handling customer data or any consumer tech company with >10M users. Once you add other requirements to the mix, such as index freshness and metadata filtering, the managed options where this is already built-in start to become compelling even at lower volumes.

Also, Pinecone (disclosure: I work there) has usage-based billing that starts at $72/month, so "paying for the technology" is not that scary.