...if I think about it, the few companies that did rewrite their codebase and benefited from this were the ones that were their own customers of their code as J

...if I think about it, the few companies that did rewrite their codebase and benefited from this were the ones that were their own customers of their code as JS and Twitter both fall in here. You're right, it's probably not worth it if you're selling "software as a product", but when you are basically an saas business or even better, you're doing in-house dev, and everything runs on your servers, you are free to even run simultaneously multiple version of your thing for different tasks.