They signed two former #1 picks and a former #2 pick so the players have talent.
They just haven't done as much as they probably should have. In most cases, injury was a issue.
The Cowboys had some success fixing broken 1st rounders before. Malik Hooker turned his career around and if just one of these guys can do the same it may be worth it.
Besides, one way to prevent expensive free agents is to make your guys better. If these new Cowboys can at least provide some competition then there is no better incentive to improve.