> This would all be a bit simpler if an indent was obligatory after
> inserting a newline in the middle of a command.

This is not such a bad idea, but it is not backward-compatible and it would be inconsistent with how parentheses works in other situations.

Developers usually learn that they should look back at least till the previous line while investigating syntax errors.  Even in C (and similar languages) it's common that errors reported on one line are caused by e.g. a missing ';' on the previous line.