I don't see it mentioned enough in the comments here, but not considering Celery as an alternative to Django + async really is the missing puzzle piece here. Aside from application-level options that weren't explored, I'm wondering whether handling some of the file IO stuff with, for instance, nginx, might be a better fit for their use case.

Once you're in the situation of supporting a production system with some of the limitations mentioned, you also owe it to yourself to truly evaluate all available options. A rewrite is rarely the right solution. From an engineering standpoint, assuming you knew the requirements pretty early on, painting yourself into a bad enough corner to scrap the whole thing and pick a new language gives me significant pause for thought.

In all honesty I consider a lot of this blog post to be a real cause for concern -- the tone, the conflating arguments (if your tests were bad before, just revisit them), the premature concern around scaling. It really feels like they may have jumped to an expensive conclusion without adequate research.

In an interview, I would not advance a candidate like this. If I had a report who exhibited this kind of reasoning, I'd be drilling them on fundamentals and double-checking their work through the entire engineering process.