Even if you were managing this for third parties, there's no reason to assume your scale would necessitate this complexity. One of my Postgres databases stores

Even if you were managing this for third parties, there's no reason to assume your scale would necessitate this complexity. One of my Postgres databases stores analytics data for podcasts. 800gb worth in two tables. It's just vanilla Postgres with indexes. P90 queries complete in 100ms. I serve >10k customers with one write instance and one read-only replica. Some podcasts get thousands of listens per second at peak times. Stripe relied entirely on Mongo for storage and queueing at a scale that you'll almost assuredly never reach. That's probably inadvisable today, but the point is that the more systems you have, the more systems you need to maintain and the more things that can break and the more places you'll eventually find bottlenecks.