Generally speaking when people start going towards qualitative generalizations such as 'soft' as an explanation it is a clear sign that they have no clue what they are talking about.
There were plenty of games where the team exhibited mental toughness over and over again.
Newman could not run as fast as he once could. You saw that watching him play Cruz and Jackson. Thats why he played off. Its pretty obvious. When you say that he is 'soft,' I say you are clueless.
Bradie James and Keith Brooking cannot run. You should have been able to see that as teams would run slot receivers, backs and TE across their zones and they could not keep up. Carter was not used to playing run defense in a 3-4 alignment. He chose wrong gaps or would hesitate. Thats not soft. Soft is just a simplification for those that cannot or will not look deeper.
Our center has (hopefully had)short arms. He would lunge to try and make contact and DT would club him to the ground. We saw that over and over again. Before the season ever began we were going with low round draft picks and UDFA. Thats not soft. If anything its rebuilding an oline and overaggression.
A 'killer instinct' is not going to give those guys speed. We need to bring in better talent that can execute the proper techniques at the relative positions.