It seems more likely that bounded rationality is at play here, where different parties only know part of the picture (and fail to bring these together and find out what would be best globally.)
This dynamic comes up often in engineering management.
Not every case of irrational behavior is caused by emotions though. And when we are making an argument that people are acting against their own interests, it may help to ponder what makes them do so. All the more when we are claiming principles and values that should be accepted by everyone.
"If you don't believe me you are acting irrational / it's because you are emotionally attached" does not seem to be an attitude that gets closer to real causes in a discussion on how to best seek simplicity, but rather a recipe for avoiding discussion or a "thought-terminating cliche."
There must be a better argument for convincing people to let go of code / clean up etc.
Clinging to code you wrote is a very natural thing to do, there are many reasons we do it, and most of the time it’s irrational.