No coach is ever going to hit every evaluation of a kid right but if you are going to mandate that they honor these scholarships for 4 years regardless of player performance then a coach should never be let go for performance reasons prior to their contract expiring. ADs and presidents should be held to that same standard which appears to be, "hey, we hired the wrong guy but that is our fault so screw it we will deal with 3 more years if crappy football."
These kids aren't being paid millions of dollars to succeed. When coaches are let go for performance, almost all of the time they are paid out for their contract, or a good portion of it. Apples and oranges.