I completely agree with all your points on significant whitespace for programming languages. Where I think it makes a lot more sense though is for structured static data. My quality of live would be worsened without the likes of Haml, Sass and YAML.
I enjoy using slim in my views though prefer scss to sass. I think for many of those tools either misindentations raise an error (like failing a yaml schema validation) or result in incorrect but observable behavior (content isn’t in a div as expected).
It really comes down to how the tool is supposed to be used and how failures are handled.
In general I think that’s the difference between a good experience and a great one: when we can have beautiful failures that help us learn.
This makes no sense.
If the arguments against significant whitespace are valid for executable code they're also valid for markup.
If it's fine to use significant whitespace for markup / data literals even the perceived drawbacks apply than it would be equally fine for executable code.