There was some fan outrage when Arraez was traded too. If you're running the team the way the fans want you to, then you're going to lose. And despite the outrage from this offseason, the Twins are 10 games better than they were at this point last season, despite playing in a much better division, and without Lewis for 2/3 of it.
Lewis did help them end the playoff drought last year, but ultimately they still fell short. MLB trades are made with prospects all of the time, even damaged ones with a much less proven track record than Lewis.