I was honestly a bit annoyed that they decided to introduce the Zeta Class and the Delta Class in Rogue One since they’re never seen again, but my suggestion seemed like the best way to make the shuttle’s disappearance from the Saga less jarring. Rogue One would have worked just as well if they had used the Sentinel in the same role as the Zeta. But anyway, it hadn’t really occurred to me, you’re right, they would basically be fulfilling the exact same role in both films if the Zeta was used in lieu of the Lambda. I dunno if that is necessarily a bad thing per-say…