Kosar was done by the time BB got there; they were 3-13 the year before and BB rebuilt and got them to 11-5 four years later.
Yes the Bills probably win 2 SBs with Brady. BB has won 4 and was close on 2 others. That's the difference.
As for Rex, as was stated in the other thread, Whaley has assembled a .500 team so why are people upset that Rex has produced .500 seasons out of them? That's what middle tier coaches do. If Rex was a top tier coach he might get more out of the talent, but the Bills aren't getting a top tier coach here, ever. The alternative is bottom tier coaches who get less out of their talent and there are plenty of those around.