You might be right about the expectations, but everyone one of those 3: Maxi, Wallace and Jack, performed at or above their career norms in 2017 in terms of scoring. Wallace has averaged 3 G and 4.5 A in an 8 year MLS career. He exceeded both of those. Maxi had a somewhat above average year, and Jack is only in his second year, and did better than his first. Expecting that a combination of the 3 of them -- even put Villa in the mix -- would make them all better than they've ever been was a dubious hope. The worst I can say about Rodney and Jack is the second half falloff.
Harrison is a bit more of a wildcard than the others because there is less of a history to go on. He might be better than he has shown, and if so you can fault him for not doing better. But this also might be who he is. And I also have to agree with what
Kjbert said below: that Jack outperformed his contract. I think you have to measure every player in MLS by their cost, and a DP slot is the most limited asset in the MLS salary structure, which is why I picked on Maxi.