The Dodgers have the reputation of never trading anything they truly believe in.
I do agree with you on the Dodgers and their reputation.
I don't recall hearing that about the Dodgers. Doesn't really match reality, either. Friedman has traded away Dee Gordon, Miguel Rojas, Frankie Montas, Yordan Alvarez, Oniel Cruz, Dean Kremer, Alex Verdugo, Josiah Gray, and Kelbert Ruiz.
I think the main takeaway is that they always have a great system because they use every resource they can to enhance the farm system and they scout and develop well. They then use the system both to graduate talent and make trades. Some guys they trade away are stud prospects that meet expectations, some are studs that fall short, others are modest prospects that explode. I don't think there is any substance to the idea that they know who to trade away and always will win trades because someone they deal is destined to fail.
Any at-the-time disappointment in Diaz was only because people felt Manny should have been worth more. Any after-the-fact disappointment in Diaz is because after a hot start to his O's career he became unable to stay in the lineup and then started really struggling even when he could. Both are valid criticisms if used within the proper context, but neither negates Diaz as a very legitimate prospect in mid-2018.