> spawn a new database every time they have a new client.

I've seen this work great for multitenancy, with sqlite (again, a single beefy server goes a long way). At some point though, you hit niche scaling issues and that's how you end up with, e.g., arcane sqlite hacks. Hopefully these mostly come from people who have found early success, or looked at by others who want reassurance that there is an escape hatch that doesn't involve rewriting everything for web scale.