Never. I'm a data scientist myself and know many other so-called data scientists. But coming from an engineering background, I pretty much agree with you. & interestingly, all of the ppl I know who claim they want to write an OS one day are all data scientist. Seriously! They don't even know what's unit testing!!!

Why would data scientists aspire to write an OS? Sounds puzzling.

I was talking from the viewpoint of the OP who was asking if data scientists were replacing programmers.

The OP is explicitly about the kind of programming work where "An entire interesting application may fit in one file.", which doesn't tend to apply to OS's nowadays.

Why would we expect a statistician/data analyst to write an OS? I'd expect them to write reports, white papers, articles, and functional domain-specific packages for R or Python. Especially in my field of healthcare, where over 50% of the analysts use SAS, I doubt we'll see any groundbreaking innovation, at best it's incremental changes via papers or sharing code.

Why would you be expecting a data scientist to be writing an OS?