Umm, WTF? You're really asking me to justify the basic software
engineering principle that modularity is good, and wanting to dump
everything in one file is one of the classic signs that your
architectural design is non-existent and you have completely failed to
adequately decompose your problem into smaller ones? Here it's even
worse, because we *have* already done that decomposition, and people
are proposing that we *abandon it*.

The classic work on the physical design of software is probably Large
Scale C++ Software Design from Lakos, and even though much of that is
C++ specific (and the technical specifics are now rather dated even in
that space), the general principles of understanding your physical
dependencies and allowing suitable levels of granularity in your
dependency hierarchy holds regardless of your development language.

Small is beautiful (but not too small, that way lies madness).