They are upside-lovers, but they don't have Marco listed in the top-10 for STL at all (because he's not). MLBPipeline just installed O'Neill as the Cardinals' #4 prospect, though, while Marco comes in at #7 for us (those lists don't include this year's draft picks, just evaluations of prospects already in the minors in Spring 2017).
So the Ms traded back and lost value because they have no pitchers, but didn't get an upside pitcher either. I think O'Neill's road to ultimate success in the bigs is dicey, but not any dicier than Marco's! Dipoto's penchant for saying, "that guy, whatever the cost" is going to hurt the Mariners long-term, because it removes trade chips that could have been used in other deals. Even if Marco works out, O'Neill was an overpay. He can't keep overpaying for everyone who wasn't originally a D-Back.