Remember when Parayko was considered one of the best young defensemen in the league? Pepperidge Farms remembers...
I don't think that his ability/role/performance since he was a young D man is at all out of line with the (non-Blues fan) hype early in his career.
This is a 2017 list where he comes down as the 20th best player under 25. He's behind Werenski, Ekblad, Jones and ahead of Lindholm, Hamilton, Klefbom, Trouba, Slavin, and Klingberg. I think he's had a better career than 2-3 guys behind him on that list and I think the quotes in the article make it clear that the projection isn't that of a Norris-level guy:
While Parayko may not reach the upper echelon of a do-it-all defenseman, his burgeoning offensive skill and playmaking abilities are more than enough to rank him at No. 20 on our list. This was his final year of eligibility for this type of list because he turned 25 before the next one came out and this was certainly the peak of hype around him as an (older) young D man.
This 2016 blurb projects him to 'quickly and consistently' be a 40 point guy and the discussion of him making Shatty expendable seems to presume that he will eventually take over more PP minutes and offensive situations.
Parayko hasn't reached the offensive production that was expected, but he's exceeded the defensive ability that was expected. He's developed into a different type of player than people thought, but I think the belief was that he'd be a really good but not great D man. During and after the first 2 years of his career, I think that he was considered as a guy who would be a #1/2 tweener and not a franchise D man. A lot of Blues fans believed that he was a lock to become a stud #1, but I don't think that was true league-wide. While a couple people behind him on the list above have surpassed him, I don't think that any of them were considered locks to be legit #1 guys the way Werenski and Ekblad were.
Parayko was the #2 D man on a defense-first Cup winner. He's paced as a 35 point guy in a very, very defense-first #1/2 tweener role on this blueline since then. His 48 even strength points are 41st among D men in that time, which is pretty darn good when you consider what his job is on this team. He's absolutely been an effective #2 D man at both ends of the ice and has arguably been a passable #1. I don't see that as a let down from the non-homer expectations. That type of D man is very valuable in the NHL.
Edit: FWIW, there is zero doubt in my mind that he'd be a 45 point D man if he got consistent PP time and was given free reign to focus on offense at the expense of some defensive responsibilities the way that most of the projections expected. I think it is better for the team when he's playing a shutdown role, but that doesn't mean he isn't capable of playing a more offense-focused role.