Good point. I think the problem has been accentuated by two factors: i) the first is that it used to be that you could find corners that weren't amazing athletes but they could be effective by pulverizing the receivers, but Polian got that legislated out of the game for his Colts, so now corners all have to be amazing athletes; ii) the second is the proliferation of extremely large and tall receivers.
The combination of the two means that the ideal cornerback is a true freak of nature like Patrick Peterson, big enough to cover big receivers but an amazing athlete at that size. It is of course virtually impossible to find someone like that when drafting 30 or below.
The other solution is to have a mix and match set of cornerbacks, some super quick and shifty and at least one that is bigger and more physical. Last year it worked to perfection with Revis as the cover guy, Browner as the big physical corner, and others to fill in. Sometimes Browner was the CB2 and sometimes not, depending on the matchup.
IMHO, the primary problem this year has been the lack of a bigger more physical cornerback that can play on the edge against big receivers. BB's first attempt was Fletcher, 6', 205 lbs, but he didn't work out, he followed a very bad year with Philadelphia in 2014 with a poor start here. Now, we are bringing in Rashaan Melvin, who is 6' 2'', 193. Not likely to be a wrecking ball, but at least he has height.
I know roster positions are at a premium, but I had my way we would continue to poach young cornerbacks from other teams practice squads and give them a whirl to see if the light comes on with any of them. If Brown is IR'd then I wouldn't mind seeing us poach another young cornerback.