Defenses are good early in the season but most good offenses adjust and become on par or better than defenses. It will be the offense that carries the team at the end of the season.
The game is a balancing act.
You have to have both a good offense and defense. The only way you can win in the postseason with one much less than the other is if the other one is a dominant one, i.e., an exceptional offense or defense, e.g., the Bills of the 90s or the Ravens in 2000. But even the Bills of the 90s could not overcome the good to great offenses and defenses of their opponents in the SBs they lost.
Even if your defense is good, if your offense lays a turd, they're going to lose. Likewise, the same applies on the other side of the ball.
The Cowboys lost against SF because the offense kept laying a turd. A good to very good defense can only hold a good offense/defense team for so long.
That's why people have issues with Dak. He doesn't handle higher level competition well because he simply doesn't have the tangibles to be more than just a good QB and he can't do that consistently. He needs the team around of him to play much better than they are capable of to compensate.