Unless your using database specific features. One of the biggest advantages for Postgres is how incredible the ecosystem is. It doesn't work for everything, but I have an OEM, multiple kinds of text search (vector, inverted indexes, trigrams), recursive and graph-like queries (though that's admittedly less of an issue if n+1 isn't a problem), row-level acls, locks, etc.

It's really nice to have all of that power available in one piece of infrastructure.

Not really a problem if you go from sqlite to postgres. Which sqlite feature is missing from Postgres?

All SQLite queries are not just going to straight up work in Postgres.

e.g handling of dates is a big one. In SQLite they are just strings(kludgy IMO), where as Postgres has the timestamp data type.

You're missing the point. Most software doesn't need to scale or solve hard data storage problems, and if it ends up having to, you can always upgrade to Postgres with minimal effort. That makes SQLite an attractive option if you won't immediately benefit from Postgres' rich features.

We're just going to be speaking past each other, I think. GP clearly stated they lean heavily on Postgres technologies (by calling out specifics like pgvector). Stating that the applications that don't need those technologies, indeed don't need them, is tautological!