I don't think it is unreasonable to expect the defense to step up and force a turnover or GT to punt the ball so the offense can get the ball back. Heck there has never been a game where GT didn't punt the ball, so the defense does know how to stop GT.

The 2009 game may be a good example of reasonable expectations for the defense. VT was ranked #4 and GT was ranked #19. Tyrod scores a TD on a 22 yd rush with 4:05 to go in the game to cut GT's lead to 21-16 (2-pt conversion failed). Any reasonable person would expect the defense to hold GT's offense and force a punt/turnover. Instead GT ran 7 plays for 75 yds and scored a 39 yd rushing TD in 1:45 seconds to go up 28-16. The offense made a valiant effort and scored a late TD to cut the lead to 28-23 (final score).

You can say that the offense should have scored more points (can't agree/disagree there). However, someone could say the defense shouldn't have given up a 12 play, 86 yd, 6:19 drive for a TD in the 3rd Q to allow GT to go up 21-10. In close games there are plenty of things all 3 units could do differently which may or may not have an outcome on the final score. It is why what a unit does or does not do on drives late in the game matter most because there are fewer opportunities to make up for mistakes late in the game.