Trevor Story wasn't under 1 year of control though and long term he can fill in Bogey's spot.
I think it can explain why Bogey stayed at SS despite being the inferior defender. Well you've got only one year left of the guy, why even bother?
Bogaerts was kept at SS in a minor part, because the heir apparent, Jeter Downs, hasn't been able to hit.. Downs may figure that out... but, all indications are that he's just not going to be the kind of hitter the Red Sox expected.. Dicky York.. the next in line after Downs.. has shown an inability to throw the ball deep in the hole.. so, the Sawx have moved him to 2B.. it's a good move, imho.. but not necessarily for York as he now has Trevor Story in front of him..
That leaves the Sawx with a choice of keeping Story at 2B (good).. moving Story back to his original spot of SS (better) or moving Story to 3B (best, especially if Devers cannot be extended) while adding a guy like Rosario to SS with the idea Rosario WILL be extended.. The Red Sox have a small crowding / glut problem at 1B (Hosmer, Dalbec, Casas and Devers if he's resigned). so, this might be a fit both clubs while providing flexibility to the Sawx:
1B: Devers, Casas Hosmer
2B: Arroyo, Story, York, Downs
SS: Rosario, Story, Yu Chang
3B: Story, Devers
While this group isn't going to wow anyone with their defense.. all of these guys can and have hit.. Nothing is imminent.. but the Red Sox, like every other club can see a value in Rosario.. an ability to extend him at a reasonable cost.. and a guy who's defense at a premium position is average to slightly above average.. no more... no less...
So.. Who besides Dalbec would anyone want in return for Rosario?.. Perhaps no one and a rejection of a deal with the Sawx, outright?..