I'm pretty sure that it's a 10-20 minute job to write a Git subcommand wrapper that checks out a work vs personal project, and then symlinks the correct config file for the repo, if someone chose to go this route.

Sure, but that's more like a solution for implementing something like in OP for if it didn't exist in git itself, not the 'why not just use per-repo config' that I responded to.

But... how is this easier than the solution proposed by OP?

It's not, but it's my response to the problem discussed by the parent comment.