For some strange reason, you think you have stumbled upon some deep dark secret that "defense wins championships".
Eventually a good team will win the SB and it comes from being fundamentally sound and especially proficient on one side of the football.
But it is not a tried and true reality that top defenses get you to the final round. It helps, but it is not consistent.
During Garrett's tenure, the final four has been composed of all top ten defenses using your points/defensive series only twice, including this past season.
2017 - Dallas was at 17.
2016 - NE (1), Atlanta (27), Pittsburgh (11), Green Bay (28). - Dallas (13) better than two teams.
2015 - Denver (1), Carolina (2), New England (9), Seattle (4) - Dallas (17)
2014 - New England (8), Seattle (2), Indianapolis (11), Green Bay (17) - Dallas (16) better than one team.
2013 - Seattle (1), Denver (19), San Francisco (3), New England (10) - Dallas (30)
2012 - Baltimore (11), San Francisco (3), New England (12), Atlanta (10) - Dallas (26)
2011 - Giants (24), New England (21), Baltimore (2), San Francisco (1) - Dallas (19)
Using your own criteria, Dallas actually has had a better defense twice. That just so happens that still was not good enough in 2014 and 2016, the two shining moments of the Cowboys franchise for the past twenty plus years.
About the only leg you have to stand on is that Garrett needs an even better defense than he has been getting. That is using your criteria that you chose.
Unlike teams that have made it with average or even poor defenses (again, your points per defensive series rule), he has never advanced beyond the divisional round. And that is part of the problem.
By saying he needs an elite defense, you are only illustrating exactly what the issue is.
He is not good enough.