Football is always evolving & adapting to the next new thing, but there's also the phenomenon of rule changes. Clearly, it's far easier to play offense now than in the 80s & 90s. It's damn near impossible to be a shut-down, scary DB any more because you can't just light up a fool reaching for a high pass over the middle any more, and you get a PI penalty if you look at a WR funny. Teams have gone more towards:
1. 3-step drops & dump off if nothing's there
2. Passes over the middle, since you can't light up fools on a high pass any more (see the emergence of the TE / WR)
What's the point? I think the value of pass rushers is going down relative to what they used to be, and the value of DBs and LBs who can cover is going way up. Look at how dominant defenses were when they had Darelle Revis in his prime. I think the next great defenses will be teams with the best CBs and LBs who can cover. This comes right after the Niners drafted DEs in the first round every draft the last 10 years (exaggerating of course).

Remember when Belichick beat the Rams in 2001 by dropping 8 players into coverage? I expect to see defenses do that a lot more. Nickel CBs are essentially in the base defense nowadays. I'm extremely disappointed in the Niners' lack of focus on the CB position the last few years; let's hope Jason Verrett resurrects his career in SF.
Niners still need a great CB or 2 and a game-changing LB to be great IMO. Side note: Kwon Alexander had below-average grades in pass coverage; good think Lynch gave him a record contract.
