Clearly the coaching staff knows that Miller-Fox as a pair is more effective offensively, because Lavi goes to it when we need a goal. The hesitance to switch the D pairs imo comes from A. Not wanting to play Trouba and Lindgren together because yikes and B. Fox's preference is to play with Lindgren. Probably a combo of both and a sprinkle of C. Schneider and Gus have been consistent as a pair all year (despite Gus being average since the first month of the season).
Brooks wrote today that Kakko succeeding in the top 6 could lead the the Rangers focusing more on 3C and D. I havent seen a single rumor from a credible source about the Rangers being interested in defense, so I hope Drury is cognizant of it and just operating under the fog of war while all the Western contenders and Tampa haggle over Tanev and Hanifin. I'd love for him to swoop in and add a top 4 guy, because I wouldnt see it as a rental. The D needs a revamp this summer, the extent of which will be determined by how we look in the playoffs.