These people reap the greatest benefit, because they are least familiar with the codebase and get the most out of having the computer check their changes make sense.

Totally agreed. I'm not saying that they would not get benefits! I'm just saying there's also a cost and that might be the difference from someone continuing learning or giving up.

I was assuming professional programmers with at least some experience - the article is about the toolchain in use by Airbnb.

I’m not sure. A newbie experiencing a truly interactive REPL-style environment might be way faster than being stuck in an edit-compile-debug loop.

Depends on compile/deploy times really. 5 minutes per cycle is painful. 30 seconds not so much. You often just compile the part you need and that can be very fast. Depends on the language too. I've found .NET turnaround times faster than Java for example.

There is a cost to breaking up your logic into various modules/packages on the file system when you could simply write all your code in one gigantic file and not worry about any of that. Just because it saves time doesn't mean it's the best choice.

Is there any situation where it'd be better to keep all your JS to a single file?

Premature extraction can solidify premature abstractions and create pressure against reabstraction.

The creator of Elm has a talk that mentions this: https://www.youtube.com/watch?v=XpDsk374LDE