Earlier quoted context omitted.
I'm aware, but I'm responding to the myth of: > Any effort in trying to bring it up to modern webdev standards would likely make the code expand significantly That's just not true, and nowhere did that person say: "Because of legacy IE support requirements", they only mentioned "modern webdev standards". Are you suggesting arrow functions, and classList aren't modern standards then?
I was that person, and what I meant was that "modern webdev standards" would include modules and features requiring transpilation, which would introduce many dependencies, all of which count into code size (you have to keep track of them mentally).
Why/where would modules be necessary here? Just because there's a feature now that didn't exist years ago doesn't mean it's the right time & place to use it. The code works as-is, a small refactor to leverage built-in functions to do some of the trickier logic parts would make this code a breeze. There's no need for something like Babel to help out with a small refactor.