IMO, 'everything in one file' is fine if the project is new or if it's a relatively small project (e.g. a library or module). It's OK for a developer to start writing the MVP in one file and then break it up later once the project start to grow in complexity and the requirements become more clear.

If a developer starts writing the code in multiple files from the beginning but it's broken up incorrectly (incorrect abstractions), then that can lead to significant long term problems for the projects which could plague it for years to come.

It's much better to have fewer, overly broad abstractions than many overly specific but incorrect abstractions.