Not too familiar with Neon other than the basics - its premise is that you use S3 as bottomless storage for Postgres and it’s otherwise the same as standard Postgres right? And this is all open source? Why are people paying? Can’t you use a cloud provider and have them host this for you?
Hosting and operating the autoscaling of the various services (compute, pageserver, safekeeper, storage broker) that it takes to make all that work is complex enough that most folks would rather not. Same as any other "managed X" service.
> you use S3 as bottomless storage for Postgres [...] Why are people paying?
It's vastly more complicated to do this efficiently than you might imagine. Postgres' internal architecture is built around a very different set of assumptions (pages, WAL, local disk etc.) than what the S3 API offers.
Neon is only scale-to-zero from the point of view of the tenants; whoever runs the software, must keep nodes running, including triple-replicated WAL storage.