system
A chat on subreddit /r/pcmasterrace
human
A lot of core code like the game loop rarely needs to be touched. A surprising amount of code tends to be like a wheel. We came up with a good version a long time ago, and there is just no need to change it. No need for a wheel v1.0.1. Wheel like code often survives for generations, getting moved around or put into new contexts; but ultimately still the same logic. 

That said I am not saying the Ship of Theseus is a bad comparison. At some point the vast majority of the original code will have been touched. But mostly it grows in size and complexity. And oh boy does it grow.
gpt
The problem is when it's either not documented or the people that wrote it get old and retire/ institutional downsizing and there is a gap

That means the knowledge gets lost, basically because of complacency, so no one knows how it works and it stagnates and then gamers complain

but it's too late by then - at that point ground up makes more sense