If you want to worry about Sanchez somehow becoming a great QB then Jon Gruden and Mike McCarthy are the guys you need to worry about getting him and neither of them is in the market for a reclamation project at this point. If Gruden gets hired by a franchise with no QB then he might take a shot on Sanchez.
Sanchez signed only a 1 year deal and still has starter aspirations. If I'm the Eagles I ship him out if I can get a high draft pick and/or good young player with multiple years left on their contract. Schitty aside St. Louis is not a bad gig. They have good young talent on offense and a very good defense. He's on a 1 year deal. If it does not work, he can always sign elsewhere.
What quarterback wants to play against the 49ers, Seahawks, and Cardinals defenses twice a year? Not Mark Sanchez, and most definitely not Michael Vick.
You also just can't plug and play a quarterbacks into an offense. Even if St Louis brings in another QB, Hill will be the best option just because he knows the personnel and the offense. It seems to take at least a full offseason for a QB to learn a new offense and develop timing with new personnel.