In an old draft of my post I called this out more explicitly. There are two things worth thinking about.

First, if you think about actually doing this in a traditional programming language, you can quickly see that this is going to result in a lot of boilerplate. REST APIs make this a lot easier, because it's easier to say something like, "change the meaning of this command, but have everything else stay the same." Functions and classes are not setup for this kind of metaprogramming.

Second, sometimes you are breaking BC because the old behavior is extremely inconvenient to provide under a new internal implementation. This is pernicious, because "I didn't like the old function name" is not a good reason to break BC, but "I can't rewrite the internals and support the old functionality" is a good reason to break BC. So you're in a situation where your mechanism for maintaining old versions breaks down /precisely/ when you would have gotten the most utility from breaking BC.