"Slave" has a meaning in the non-computing world as well and the two concepts diverge. Which is my point. Why is the term "variable" good, it's because it can change, it's variable it describes the concept well. When programming specialty lingo does not diverge from non specialty lingo it makes the concepts easier to reason about and work grok.

The old term is confusing to newcomers, and changing it to something else would not break the brains of any tech oldtimers.