I don't think it's just about collisions, in fairness. There's also the aspect of managing user permissions by schema, searching for tables, and so on.

Obviously this only becomes super-useful in pretty sizeable projects, but I tend to consider using schemas a good habit to be in.

Have you ever tried using them before? You can set your search path, you don't need to type out schema.table. Your objects are already in a schema anyways, it isn't like you type out public.table all the time. What are these "lots of downsides" you leave unspecified? And how do they outweigh the benefits of grouping database objects together for both namespacing and permissions purposes?