code written by an expert today may be modified to incorporate new features 10 or even 20+ years from now by an entierly different group of developers
Please name me a single traditional game engine that has 20+ years of life in it.
World of Warcraft is one of the longest running major games out there today, and it hasn't had it's 9th birthday yet.
Other long-running engines include the annual sports games like Madden that is 25 years old and FIFA that is 20 years old, but those have been rewritten many times over their lifetime.
It is true that certain long-running libraries and components are deserving of unit tests. I haven't seen anybody write that they shouldn't have unit tests. But other parts of the game are short-lived and the code is effectively dead when the disc image is released to manufacturing, and these would be foolish to write unit tests for.
I never mentioned games, the post i replied to was talking about business software.