If the chosen people were at fault because they failed to abide by the Old Covenant then why.................?

To start with, the replacing was done by a group who wanted nothing to do with the covenant. You are right that there is nothing wrong with the "old one", it still stands. If one is willing to give up brother and sister, mother and father, habits and selfish wants in trade for the way to righteousness there is much waiting for them. It really wasn't an issue of blame or something being wrong. The chosen people were told from the start that it would be nearly impossible, they said they could do it. They were warned again and again that they were not going to make it, and told when they were slipping as well as given many many opportunities to come back when they had utterly failed. Those options are still there also. The fact of the matter is that when people got so far from the plan that they had no idea what to do anymore, confusion set in and the entire story was rewritten.