As someone who's liked using DI frameworks, I'm curious: if you do it manually, as your codebase gets large, doesn't it get harder to "plumb" a new object that needs to get used somewhere at a deep level? This seems like it'd get more unwieldy when refactoring.

I've never worked in a large codebase that did this manually so I'm not sure what it looks like. The large codebases I've seen that don't use DI have used something like a service locator, singletons, or constructed everything where it was needed (and used extensive mocking framework functionality for testing).

All object creation is in one place - main. So no, I should never have to go deep anywhere.

This is my take as well. Plus, if you are plumbing something that deep, the odds that your design is total shit is approaching 100%.

Manual DI reveals problems like this way earlier.

DI frameworks, IMO, are some of the most useless around. Solution seeking a problem.