I mean, sure start with a unified schema file until you have a production release... deploy, populate with placeholder data, etc... but once released, having a file for each set of changes isn't a bad thing.
Also, the management tools you can have single files for each view/sproc, etc... it's just schema migrations you need to take care of.
This only works if you don't care about being able to auto roll back DB changes without making a new commit, cause Postgres doesn't have a declarative DDL.