It wasn’t even close offensively and more importantly projecting for the future. Mets would laugh at the A’s if offered Chapman for Alonso. This reminds me of Mientkiewicz blocking Morneau and people saying you couldn’t discount defense. While 3B is mildly more important than 1B, some of the factors are the same. Chapman is limited by some physical characteristics that won’t change over time. With age his range will decline. And unlike 1B it’s not exactly like you are putting scrubs out there as the replacement defense. It’s not nearly as important as say CF defensively, which Buxton impacts more as defensive CF than Chapman does at 3B. For Alonso, it is hard to replicate that type of offense.
I don't get why you keep bringing up Alonso. Is someone suggesting the Twins have a player equivalent to Alonso who they should trade for Chapman? I don't see anyone suggesting they do or they should.
Chapman is 26, has a really good bat despite playing in a pitchers ballpark, great defense and draws a ton of walks. That ability to draw walks is a big safety net to help prevent him from going belly-up; he'll get on base a lot which means he likely won't slump. It's why Rosario, Schoop and Cron can go ice cold, while Polanco, Kepler and Cruz can ride out a hitless streak and still not kill the lineup.
With his excellent on base skills, well above average power and elite defense, he looks like Scott Rolen, who was rarely a sexy player but ended up one of the best 3B of all time without anyone really talking about it.