Definitely the case in 2014...5 of last 6 opponents finished in AP Top 25. Not the case last year; only 2 of last 7 finished in Top 25. This year, 3 of last 6 (GA, TN and UofL) will likely open in pre-season Top 25.
On this depth discussion, sure, you gotta have subs for injury purposes. But for basic, in game substitution there are 6 positions where, barring in game injury, you simply do not see much substitution at all, the O-line and QB. If you play with a lot of TE sets you can probably get away with playing that position the entire game as well. The common denominator is you "get tired" when you run a lot. That means everybody on defense, WRs and RBs on offense. But you will still probably find some defensive players that play virtually every (meaningful) snap.
As a guess, I would say first team players at such "running positions" play about 70% of the snaps (in a close game). And there are damn few places where the #2 guy is as good as the #1 guy. But it is not like you substitute a whole new team at once. And remember, in today's game, A LOT of those substitutions are "player packages" dictated by game situation and not a reflection of weariness.
Depth is important when replacing an injured starter but I think over the course of a game it is not as significant as a number of people are making it to be. Especially from the standpoint of "wearing down" at the end of the game. FWIW, in 2015, UK's opponents possessed the ball an average of 1:30 more than the Cats and ran 2 more plays/game than did the Cats. I'm not sure that differential is enough to create a tired team situation. JMO, of course...I've never been on a college sideline during a game to see how it really unfolds.
Peace