Here's another way to look at it:
Last season, all of the CFB teams with similar defensive stats to Nebraska (we were 13th in Total D) won at least 8 games. We were the lone outlier.
It's not crazy to expect we'll field as good, if not a better, a defensive team this season. That calibur of defense can usually be counted on to hold its opponents to no more than a TD & FG or 2 TDs.
Last year's offense (plagued with all the injuries and an avalanche of turnovers) averaged 18 points a game. FYI, a top 60-ranked scoring offense (generally speaking, the top half of CFB) last year averaged four TDs per game (28.2.) By comparison, our offense ranked near the bottom in the country.
SO....if this year's offense can improve to a 'mediocre' average of 28 points a game (or, still my beating heart, BEYOND), the skies the limit with this team.