I also do not consider websites or smartphone apps to be serious software, despite the huge amount of work that happens to be done there.

You know the rule, 99% of everything is crud.

I find it hard to understand how you can believe something like Dropbox isn't "serious production" just because it involves websites and apps.

the program that does the synchronization of files in the background is.

Last time I checked, that part was written in Python...

And the fact that they're spending so much time on it and were afraid to rewrite it tells you all you need to know.

Rewriting it in C++, leveraging the platform-specific APIs for disk I/O (sync_file_range on Linux, not even wrapped by a Python module) would yield not only much better performance, but also much higher reliability.

As it is this daemon is a recipe for page thrashing. Not that I would expect a Python dev to actually know how a kernel works.

Most Kernel devs are Python devs.

You ship C code which is extensively tested by Python code.

Python is simple to use, got good C interop and the performance of your test frameworks hardly matters.