At this point I think you make that decision when Kepler, Vargas and Sano are all pushing their way into the lineup for AB's. As of right now Vargas isn't hitting, Sano has never played 1B and Kepler is still in AA. The only sunk cost is Mauer's salary this season. The $23M per for 2016, 2017 and 2018 are 100% guaranteed future costs. Mauer's OPS+ of 90 this year could be a whole lot worse for all the talk of getting benched and flat out retiring.