I also think you're forgetting some of the offensive injuries in 2015, particularly at receiver. Sure, Doctson didn't get hurt until late, but there were games early in the year where he was the only healthy starter at WR. Listenbee, D Gray, Slanina, Porter all missed time.
Also, Jamelle Naff and Joey Hunt got hurt at year end.
That's the problem with this year's team I guess. In 2015 we were so good that injuries didn't derail the season. This year we're not good enough to sustain injuries and still win a lot of games.
I think some of that may have to do with the nature of offense and defense. Yes 2015 defense was one of the worst of the the Patterson era due to it being a rebuilding year combined with losing 5 starters over the by game three (James McFarland, Sammy Douglas, Mike Freeze, Kenny Illoka, Ranthony Texada).
However, that 2015 offense was amazing. A senior laden Oline (Vaitai, Naff, Hunt, Foltz, Noteboom), Boykin and Doctson in their prime, Aaron Green at RB, and a stud freshman slot receiver in Turpin to top it all off. There were a lot of games where Boykin and Doctson went straight beast mode and carried the team to victory, they were practically unstoppable. SMU, Tech, K State games were won with unbelievable Doctson/Boykin plays. No matter how bad our defense was, those guys were winning games.
Our offense carried the team that year.
The moral of the story is yes defense wins championships, but if you have a great offense and a terrible defense, you can still win a lot of games (2015 squad). But If you have a great defense and a terrible offense, you are going to lose a lot of games (2018 squad). Great offenses are always going to do their thing even if the defense is letting them down. Great defenses on the other can lose motivation and get tired, worn down, and eventually demoralized if their offense can’t do anything.