He proved he could hit and yeah then he regressed but I bet he learned a bit in AAA to be at least good enough a better bet than Duran vs LHP. We'll never know for sure though.
Said that IMO guys like Campbell, who have a legit talent, have nothing else to learn or prove in AAA. He will always hit 800+ at the end of the day there. What he needs is to face MLB pitchers as much as possible. It may take a year or two to adjust and I can live with that. There's a reason they extended him. If he turns into another Bobby Dalbec, it's baseball. Some prospects pan out (actually very few) and some not. Look what happened to Duran. MLB experience helped him to be what he is now. I want to see if Campbell has that resilience. Talent gives you the floor and character the ceiling.