Fangraphs did a study. If you have a low OBP team, who hits for power, you are better off adding more of the same kind of player, than a high OBP player. Mauer might make 2 FWAR this year, but I doubt it. I have no interest in comparing him to SS, C, and pitchers ("league average hitter"), but only their other options and other 1B. It seems clear he can't play 140 games and be effective anymore, so if you have him, you need someone that can play 1B 40-60 times. With a tiny bench, not sure how that works really. The real question is, do they have someone that can play 1B or 3B or RF (if you move Sano or Kepler) that is better than Mauer in 2019. Given that he is league median for 1B (and you'd assume he continues to age and his hitting and baserunning will get worse, that's what age does), I'd hope they can find someone that is league median or better that will be here for more than 1 year. Corner OF FA+Kepler at 1B is probably > Kepler in RF and Mauer at 1B by 2019. If not, then Kepler hasn't progressed and isn't all that good a corner OF compared to other corner OF.