Here's a couple problems I have with this line of reasoning.
1. If pitchers have no control over BABIP, how do you explain Chris Young, Dave Stieb, Sid Fernandez, Charlie Hough, Matt Cain, etc... who have consistently maintained BABIP rates below average. See http://rotosynthesis.rotowire.com/Is-Pitcher-BABIP-Luck-BBD3653.htm
2. History may show that it is more likely than not that Sanchez will not dramatically improve his walk rates, but it doesn't say that he CAN'T. There have been guys who have done it, and there's no reason he can't. His walk rates since the middle of last season have been dramatically better then his previous rates, so it's possible that this will continue.
To me, until he proves that his numbers since the end of June 2014 were a fluke, he deserves a shot.