With all due respect to Cashner, he's 27. This is the point of his career where he's suppose to be his absolute best. Rizzo is 24. A lot of quality 1B don't even see the majors for the first time until 24. Votto had 89 PAs as a 23 year old and the following season was his first full year for example. And as I've said before, he's not even playing that bad. His .241/.327/.413 is lower than you'd like but a .327 OBP is still pretty good and his average seems to be heavily influenced by BABIP last season. And again, we've likely not seen the best Rizzo that he will be as of yet.
Additionally, the cubs pitching really hasn't been that bad with the exceptions of Jackson and Villanueva. In the case of Villanueva, he's likely only starting because of an injury to Arrieta and in any case he's a 5th starter. Sharks ERA is 1.29, Hammel is 2.63, and Wood is 2.92. Jackson is obviously the disappointing one as he's who they actually gave decent money to. If Rizzo is "losing" a trade then I'm fine with it. Imagine where the already bad offense would be without him. Meanwhile, they've managed to scrounge up 3 pretty decent pitchers for next to nothing in FA(Maholm, Feldman, Hammel). If Cashner turns into a Cy young level pitcher then perhaps it was a mistake. But if he's your Garza/Shark level pitcher in that he's either injured or inconsistent with flashes of brilliance then he's not that hard to replace.