People like to talk about the lack of star hitting power on this team compared to the Cardinals, but Adames' Statcast numbers this year are virtually identical to Goldschmidt's and largely better than Arenado's. Plus, Adames plays the premium defensive position on the infield quite well. I'm NOT saying that Adames is better than those guys (they are both likely future Hall of Famers), but it is quite interesting nevertheless.
In addition, after a blazing start in April, Arenado has cooled off and is hitting more in line with his expected numbers. Meanwhile, as a result of his insane hot streak, Goldschmidt has significantly surpassed his expected stats. So, Goldschmidt will likely start to cool off soon, and the Cardinals will have to fill that lost production somewhere else to keep winning at their current rate. They are unlikely to get that from their three rookies, who are also currently hitting way above expectations, nor can they expect to get it from 40 year olds such as Pujols and Molina. That narrows the pool of candidates to O'Neill and Carlson, who are both due to return from injury soon. O'Neill's problem, as you say, is his huge swing and miss tendencies which bear a striking resemblance to those of Hiura. Last year, he was able to combat this by hitting the ball extremely hard when he did make contact. However, this year, his hard-hit percentage has taken a nose dive, which has led to him being statistically one of the worst position players in baseball, even worse than Lorenzo Cain by bWAR. That is a huge problem for the Cardinals, and one I'm not sure is easily rectified.