I love Dex, but I think people don't realize how much the Cubs and their insane positioning group creates the defense they have. His defense only became passable with the Cubs because his range went from massively negative to slightly positive and the Cardinals don't have that system for him to walk in to. While his bat was good here, it was also incredibly streaky and it's always easier to get out of a slump when you're surrounded by great players because teams, even when you're struggling, still have to throw you strikes. He also derives a lot of value from baserunning and I'm not sure I trust a guy at age 32+ to give me highly positive baserunning value while playing every day in the outfield.
I love Dex, I think he was insanely valuable for the Cubs these past two years, but to give him 5/80 and give up a first round pick seems insane to me. Cardinals basically need Dex to be a 3+ WAR guy every year of this deal and if he's not that because his defense slips + time off due to an injury like he has every year, this deal can get away from you quickly. Someone is going to come in and tell me that $8 million / WAR is the current market rate and while I think I have even used that number in the past, I think that number is far too high to evaluate any deal and I was wrong when I did. Looking at what St Louis is paying for, I think they need Dexter to be about 15 WAR or else they've drastically overpaid the market (especially losing a first round pick).