It's more than a matter of Croshere being overpaid.
It's one thing to say, "OK, we can see now that we overpaid him, but he's a good guy and does his job without complaint -- plus he's virtually untradable -- so we'll keep him."
But it's quite another to choose to pay $18M for that mistake both of the next two years (which includes the luxury tax payments his contract requires), when you have the option of waiving him and only having to pay half that amount.
At the end of the day, the Simons are running a business; and as much as they, Donnie, Larry, and Rick may like to have Croshere around, they won't make the mistake of over paying for his services twice.
If he doesn't opt out of his contract (yeah, right), get traded (almost as unlikely), or retire (ditto), he WILL be waived.