I'm not buying this argument. Belicheck had Kosar and Testaverde in Cleveland and went 5-11 with Bledsoe in his first year in New England. Those QB's were talented. Not an immortal player like Brady but much more talented than Henne. I'd bet that Kosar, Bledsoe and Testaverde combined threw for well over 100,000 yards.
Kosar was total garbage at that point and his decision to get rid of Kosar was part of why he got run out of Cleveland.