You don't need to count whitespace for nesting:

    test:
        something:
        complicated:
            {even: { more: { levels: {can: { be: {achieved: {like: {this: true } } } } } } } }

Whitespace nesting also doesn't seem to be a problem for Python programmers. I don't prefer it myself (especially because of the tabs vs spaces problem) but it's not good or bad.

Extra whitespace before 'something' in your example would cause an issue.

> Whitespace nesting also doesn't seem to be a problem for Python programmers.

It does cause problems, IDE's mitigate the issue, the same as they do for YAML. However YAML is supposed to be a human readable data-serialization language and If you need an IDE to make it human readable then that defeats the point.

I can't imagine ever being convinced that having to 'count the number of invisible things to determine what something is' is human readable even if you can use other lines as reference points.