Your suggestion has a different meaning than the sentence here.
The original sentence/translation is written in the past perfect tense, "hadde vært" = "had been," which indicates something took place/happened (was completed) in the past. E.g., Erik tasted the cheese, spit it out and gave the remainder to the dog. The next day, his mum asks after the whereabouts of the cheese. Erik replies, "I threw it out because I tasted it and it had been disgusting."
"Would be" is used to speculate/hypothesize about something that has not happened. E.g., Erik's mum says, I bought that Gorgonzola because I thought it would be delicious served with red wine, pear slices and bitter chocolate!"