I think what's unstated here is an assumption that Dozier will perform again like 2016, and not revert to 2015 and before. If the latter happens, a similar "gash" in the offense occurs whether you trade him or not. Polanco, through a different mix of skills, has the potential to approximately replicate 2015 Dozier right away, and also has some further upside. Instead of just letting Dozier revert, which is the $64M question of course (it could be he's reached a new permanent plateau), a trade buffers that risk by acquiring some kind of significant talent at a different position such as starting pitcher. And, I keep repeating, Polanco isn't a long term answer at SS.