That logic doesn't hold up. The overhead in learning your way around a Google or Microsoft-sized codebase is much larger than learning a new language or framework.
That's no surprise. Both of those companies have many individual codebases that are larger than any web framework. For instance, last I checked, Django clocked in at around 60-70k [0] Python LOC. The Windows kernel source, IIRC, is over 1M LOC, and, obviously, much lower level than Django.
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[0]: Also, IIRC, 20-30k of that is taken up by the ORM.
