Earlier quoted context omitted.
If Go upgrades itself with support with generics but break backwards compatibility in a horrible way and the breaks cannot be caught at compile time, people will be complaining and resisting upgrade just the same.
And what should the Go team do if the only reasonable way to add generics is to break backward compatibility? Just rip the bandage off? Stagnate? Abandon the language and start a new one?
A key point you ignore in my comment is the "caught at compile time" part. If generics breaks backward compatibility, but the breaks can be caught at compile time, upgrading a medium codebase may take only an afternoon, and that would be reasonable commitment. Huge difference from the Python 3 scenario.