Sanchez may hit well enough to provide some real offensive value at DH, but good hitting DHs are much less valuable than equally good hitting catchers. If Sanchez became the full time DH due to his defensive shortcomings behind the plate, this would also limit the Yankees ability to rest other players by using them at DH and limit their overall roster flexibility. This means that for the Yankees one of the major things they need to do before the 2018 season starts is to work with Sanchez to fix this problem in his game so that he can remain their primary catcher. Sanchez needs to work at this throughout the postseason because for him the stakes are very high. A Gary Sanchez who fields his position to well enough be a full time catcher will be on more winning teams, an occasional MVP candidate and be much more highly compensated than Gary Sanchez the DH who will have have a shorter career as a good, but not great, DH.
This is a decision about personnel, but it is also a decision about philosophy. If the Yankees believe that they must win the World Series every year, then the logical thing to do is to bring on Napoli or Pierzynski. However, these players are 31 and 35 years old now; and Napoli has never been a full time catcher. Signing an older player, particularly if that player will quickly migrate to first base or DH, as Napoli would, would raise many problems for the Yankees, especially as both these players will likely end up with contracts of three years or more, but turning the job over to some combination of Whiteside, Cervelli and Romine will damage the Yankees' chances in 2013.