But I haven't heard anyone mention any side-effects along the lines of "oh, and people on this drug can never learn anything or have fun ever again", and you'd think somebody would have noticed

Brain interventions are rife with cases where "somebody would have noticed" failed. For example, it was a long time before anybody noticed that people with frontal lobotomies had significant personality changes. And there is a famous surgeon, Ben Carson IIRC, who has done hemispherectomies, in which he removes one-half of a young child's cortex. He claims this has no long-term harmful effect. I think he's not looking hard enough.

(Also, the brain has numerous learning systems; opioid receptors are just one type. Even for a particular type of stimulus, learning and forgetting can use different systems, as they do for fear-learning. I would think you'd need to excite the forgetting system rather than knock out the learning system in this case.)