For prose fiction, I don't see much here that is going to drive people away from Indesign. Designers and typesetters aren't really interested in doing that kind of work in a system that would require generating new output for every correction. Even relatively simple tasks like balancing columns in a spread would become a pain.
The whole point is you shouldn't have to manually balance columns in a spread. Obviously a round trip from source to output to fiddle with balancing every spread would be tedious. But if you can say "For this content I want to use spread X defined as this shape and balanced in this way" and then be able to fiddle with your content and have your spread always work, then where is the tedium? That's what SILE is supposed to do.
Many publisher's workflows involve a round of content editing bouncing a word file back and forth, then a period where a typesetter uses InDesign or similar to lay it all out, then it goes to press. You can't keep copy-editing after the designer takes over. With a workflow using source documents in Markdown and typesetting handled by SILE I am able to allow copy-edits to book manuscripts up until minutes before going to press.
