Team success doesn't always equal quarterbacking success. Those defenses were outstanding. Sanchez was a reasonable game manager his first three years in the league (his third year being the best but they went 8-8 as the defense wasn't as good). After that he disappeared. Sanchez was considered a better prospect than Geno Smith and still lasted only 4 years as a full time starter in the league. Iirc, Sanchez's best stretch was with the Eagles.