Well.
The Roy Williams tackle wont take away the big DB's. I feel its all about discipline. You can have a big guy who is diciplined, who's just as good as the smaller guy.
Remember, the CB's are still allowed to put thier hands on somebody at pthe line of scrimmage. After 5 yards, they're not allowed to. The big guy will disrupt the reciever at the line of scrimmage while the smaller guy will stay back about 5 yards.
Both are still viable strategies.
But as you say, you still need that SS that can knock people out (but not bust knees now).
I remember Roy Williams changing the face of the individual game by knocking a WR out cold. Next thing you know, that WR wont catch any balls near Williams in fear that he might die from the hit. You need that type of player to dictate the opposing offences.
The FS i feel is the guy who never lets anybody get past him. Ed Reed is a very good example of it.