Parayko has at times been an elite defenseman in the league. He has not recently. His back problems no doubt played a role in that, though there was some slippage in his game before that. But he's more likely to become an elite defensive player than, for instance, Faulk, who at his best has been an average defensive defenseman (but an above-average offensive defenseman). So when I say Parayko has a potential upside, I'm saying that Parayko has played at a high level, as recently as just before the pandemic, and it is possible to return maybe not to the highest level, but to being very very good.
I've said it seems weekly in this chat: Forget about Parayko as an offensive player. He showed promise early, but really, not since his second or third season, has he been anything more than average. I'll use the analogy I've used many times before. Dmitrij Jaskin scored a bunch of goals in his rookie season and everyone thought he was going to be a solid offensive talent. But that season was an anomaly, and it blinded everyone to the fact that Jaskin was a very good defensive forward because all anyone saw was a disappointing offensive player. Parayko is a good defensive defenseman, probably the best the Blues have. (Calle Rosen has put up good numbers this season, but the two play in very different situations.)
And as Berube said about Parayko earlier this season, he kills plays, not players. He uses his hockey smarts to stop plays without slamming a guy to the ice. If he goes, Blues fans will miss him.