The first attempts to justify stagnation and close-mindedness. It allows for people to continue to make the same mistakes time and time again. It often used as a way to absolve the person saying it of responsibility. Certainly, there can be good reasons why something may have been done the same way for generations; however, having always done something cannot be the sole justification for continuing to do it. In the Gospel, the men who bring the woman who committed adultery to Jesus are trying to trap Him in the laws of their forefathers. Jesus neatly forces them to reexamine their actions and escapes the trap laid for Him.
The second statement is usually the case of poor memory or a lack of information. Most people tend to look at the past through the rose tinted glasses of nostalgia, conveniently avoiding notice of the problems that existed. Certainly, there are some aspects of the past which were grand and wonderful, but trying to recreate that only forms a crude copy of the original. Rather, people should move forward to help create a grand future with wonders of its own.
God forgives us of our past failings so that we can move forward in His grand design. While we should learn from the past, we need to look to the future. It is there that God's perfect plan will be revealed.