In every season since 2014 and the advent of the CFP, with the exception of the 2016 season, when Clemson won the title, the team with the higher rated total defense won the National Title game.
2014 Ohio St - 19 vs Oregon - 89 - (Alabama - 12 and Florida St - 63)
2015 Alabama - 3 vs Clemson - 10 (Michigan St - 26 and Oklahama - 39)
2016 Clemson - 8 vs Alabama - 1 (Ohio St - 6 and Washington - 7)
2017 Alabama - 1 vs Georgia - 6 (Oklahoma - 67 and Clemson -4)
2018 Clemson - 5 vs Alabama - 16 (Oklahoma - 114 and Notre Dame - 30)
In only 3 of the semi final matchups, has the team with the worse defense won the game, both semis in 2014 and Ohio St over Clemson in 2016.
The average ranking of the national champ is 7.2 and that is skewed by Ohio St's ranking of 19. The average ranking of the runner up is 24.4, again skewed by Oregon's 89. The average rank of all 20 teams that have played in the CFP is 26.3, if you take out the bigges outliers, Oregon 89 and Oklahoma 114, the average drops all the way to 17.9.
As I have said before, offense matters, but when you look at Oklahoma and Oregon, Oklahoma has failed to make it out of the semis and Oregon was lucky enough to play another team with a below average defense in the semis and was then beaten by 3 TDs in the final.
Defense still wins titles.