imaging your application contains of two pieces - somewhat simple crud, that requires to respond _fast_ and huge batch processing infrastructure, that needs to work as efficient as possible, but doesn't care about single element processing time. And suddenly 'the sane thing to do' is not the best thing anymore. You need different technologies, different runtime settings and sometimes different runtimes. But most importantly they don't need constraints imposed by unrelated (other) part of the system.
You're absolutely right. My comment is towards the view that using a single language for a certain Project Backend is a bad thing per se. The online vs batch processing is the golden example of domains that should be separated in different binaries, call it microsservices or services or just Different Projects with Nothing in Common. Going further than that is where the problems arise.
imaging your application contains of two pieces - somewhat simple crud, that requires to respond _fast_ and huge batch processing infrastructure, that needs to work as efficient as possible, but doesn't care about single element processing time. And suddenly 'the sane thing to do' is not the best thing anymore. You need different technologies, different runtime settings and sometimes different runtimes. But most importantly they don't need constraints imposed by unrelated (other) part of the system.