Flamebait. You have cleaner and better examples of a world-class kernel and database engine? You want to do rocket surgery, you gotta bend a few scalpels.
FreeBSD is generally considered to be a cleaner and better example of world-class kernel. MySQL codebase is OK, but isn't fantastic. In general, all mature large systems come with a can of worms, and it takes a certain kind of person to deal with that. Legacy code is always less fun than a blank slate, unless you have certain mindset.
Fortunately, we get to deal with both world-class legacy systems, and write some blank slate code. We get the best of both worlds :)
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