> Yes splitting up source files is trivial.

It's not trivial. Have you ever had to maintain a project across multiple versions? .NET is even crazier, it supports multiple versions across multiple projects with a lot of shared code that lives in different repositories.

If you split the file you cannot apply patches to older versions or different projects anymore. Merging will be a nightmare and the change history will, while not technically lost, be very hard to follow.

Try it for yourself. Make a git repository, a large file, two or three LTS version branches, and then split the file up. Now try to apply a patch you made in master to the LTS branches.