A couple of years ago the Cowboys had Byron Jones, Lewis, Awuzie and Brown at CB. Now they have Diggs and Brown and some cast-off from the Jets. In 2016, the Cowboys had the best offensive line in football, arguably. In a few short years it became a source of concern. The problem isn't drafting. They managed to build strengths through the draft. The problem is keeping those strengths while building others. It takes too long to build an entire team through the draft. Free agency is a tool to help with the rebuilding process and the Cowboys are one franchise the simply refuses to use it.
Now the Cowboys will be forced to use multiple draft picks to find STARTING caliber CBs because they could not re-sign the ones they had. In the meantime, the DL is weak, the OL is injured and aging, the RB is getting slow, and S remains the problem it has been for years. This is why other teams use free agency not just to fill the roster, but to fix holes in their roster.