Really great work you are doing there! :)
One big reason why I always remove unused/old code is that others might find it and build up on it or (maybe even worse) use the underlying patterns. The old code might be written in an old pattern that you are currently trying to move away from. But if people see it all over the place they might adapt to it. I have seen that a lot, so I am totally with you when it comes to removing legacy code :)