This feels like a good area for tooling (editors, source hosts, SCM extensions) to improve experience. I don’t always mind large source files (and sometimes may prefer them over large file system hierarchies), but the can be a pain to navigate in some circumstances.

As an example, making several related changes in very different parts of a file, where you need to cross-reference between them. The changes themselves might be small, but it’s a huge cognitive burden to alternate/iterate through them. I’d love to have a view which temporarily projects those targets as if they’re isolated files without changing the actual structure on disk. I’d love it so much I actually do this manually for a lot of tasks, creating temp files to prepare edits for related areas of code. But then I lose a lot of the benefits of tools which understand what’s being referenced. It would be great to just type a quick command (or click or whatever) to say “don’t refactor this function to another file, but let’s pretend you did, for a while”.