Sure, I'll back off the hate until the fall... but my gut feelings still have me thinking he's not gonna come close to earning that pay. (and no, I didn't have a better offseason aquisition choice) The wife is a Cards fan, so it's less easier to handle as well.
People need to understand value a bit better. I don't mean this in a condescending way either it's just I think people don't understand how teams value players. So, as an example here, there's 30 teams who each have 5 starters. If you look at the top 150 pitchers by fWAR last year they range from 0.6-7.7 which was Sale. I'd argue you probably could cut off the #5 starter because a lot of teams aren't going to have 5 healthy starters so the 5th guy tends to end up being either replacement level or a guy who doesn't pitch enough to be significantly better than replacement level. So if you scale that back to the top 120 guys or top 4 guys on every team that range is 1.0-7.7.
Teams are readily willing to pay $9-10 mil a year for a Jason Hammel and hope he's that 1-1.5 win pitcher. Darvish is making $25 mil this year and $20 mil next year. So, you're expecting him to be some where between a 2-3 win pitcher based on his salary. And assuming he doesn't opt out after 2 years the following 4 aren't bad at $22, $22, $19, and $18 mil. What's a 2.5-3 win pitcher look like? Assuming he's healthy and makes 30 starts, Mike Leake. That is the career 4.04/4.15 ERA/FIP Leake who has a career 6.13 k/9 and a 2.16 bb/9.
In other words, if Darvish is healthy he's going to be worth what they are paying him. That's not really even open for debate. It is a similar story with Heyward. The real discussion isn't whether those players will "earn" their money. The discussion instead should be about opportunity cost. That is to say if you had someone else rather than them would that person be giving you more. But even if the argument is someone would rather have Arrieta, he's making $10 mil more over the first 2 years and there are likely issues with the luxury tax because of that on top of the fact he needs to be roughly half a win better both of the first 2 years.
Long story short, I wouldn't sweat it. There's really no way for Darvish to be financially ruinous. It's more a question of whether or not he helps you presumably when the cubs make the playoffs. And while people will only remember the WS where he wasn't so great, he totally shut down a very good cubs team in the NLCS.
