But how often would you actually have to rewrite an entire 10-20k LOC project at once?

As you need to add functionality, you can opportunistically find ways to improve the codebase toward a better/more maintainable structure.

I have several large codebases I work on which probably get totally rewritten every 18 months or 2 years, however incrementally.

If you have a working project and you think you need to throw it all out and start from scratch probably it's worth a second thought.