You can have it both ways. They can support the old version by providing security and stability patches without adding the new, computationally expensive features to the old phone.
They can at least allow downgrading to old iOS. I don't really understand their reasoning. I can decline upgrade request for years, but if I hit "OK" once, it's game over. Either upgrade automatically and don't ask me, if you're so security-consicous, or allow downgrading.