> "Master" didn't really make sense. It was supposed to mean one thing that controls another thing (the slave), but nothing ever works out that way in reality.
I always thought it took, pretty directly, the 1st meaning in https://en.wiktionary.org/wiki/master_copy, which does fit the ultimate definition as it's the copy all of the edits follow from until they are merged at which point they become the new master edit and the following edits become based off of it. Same of branches, just on a large scale of edits.
The problem with master in that context is that a master can get confused with a golden master.
That's why one is called the golden master and the other is a master, not much to confuse. The "golden" is what signifies the unique meaning, i.e. the final, that both are masters is an accurate takeaway.
Main (or most any other common primary branch name) can have equally trivial confusions that are not actual problems. The endless debates about branch names didn't come about because people were so confused we needed something else to stop the madness, they came about because people were convinced they could find a problem to need to fix. Any of these branch names work equally fine, it really doesn't matter and there is no deeper logic needed to justify why we switched.