I don't think the two can be lumped together this way. As you say, Nunez is a poor defensive 2B, but Holt's always been fine there--nothing special, but he won't hurt you. By both advanced metrics he's been basically league average for his career. With league average defense you'd ideally like a little more offense than even good Holt provides, but if he's a stopgap there, he's a high-quality stopgap. The Sox should be fine going into 2019 with Pedroia as plan A, Holt as plan B, and some combination of Nunez, Lin and (if warranted) a midseason pickup a la Kinsler as plan C. I don't think it's worth giving up talent or LT space to improve on that--certainly not unless/until Pedroia moots plan A and frees up payroll by retiring, and it's hard to see that happening before ST at the earliest.