To be clear, I don't believe that Mountcastle should be on any "no-trade" list, like Holliday, Mayo and Basallo. In fact, I've said several times that I think that Basallo could be his ultimate replacement by 2025. And, I think Mayo fits best in RF with Kjerstad belonging nowhere near the infield grass.
I just think that this board generally undervalues Mountcastle. His career OPS+ is 114. That's not average. It's 14% above league average. His Statcast numbers are outstanding with all of xwOBA (81 percentile), xBA (80), xSLG (89), Avg Exit Velo (83), Barrel% (79), Hard Hit% (70) and Sweet Spot% (66) all well above average. His Sprint Speed was 73 percentile, very high for a first baseman. Even his K% (43) and BB% (41) were close to middle of the pack. Contrary to public opinion, players with those numbers don't grow on trees.