You could argue either side could win in a JD for Devers trade. From the Red Sox perspective, 3B is a big weakness, JD solves that for a minimum of this year and next, when they've got to be thinking about challenging for the World Series. Devers may never make it to the Show as a regular, and the odds are against him ever being MVP elite worthy.
Conversely, JD's inevitable decline could already have started, and Devers could turn into an all star every year from 2018 to 2028.
That's just a risk you have to take when making trades, it's rare you get a Shelby Miller type one sided heist.