Right now they have 3 solid starters - one who is aging (Lester), one declining and almost definitely leaving (Arrieta), and Hendricks coming back from injury. That leaves 2 for next year. They'll have to rebuild their rotation for next year with 2 additions. Quintana is controlled through 2020, hence his high value and return for the Sox. The Cubs can't put all their hopes for their future rotation on finding 2 FA starters this offseason. Might as well start the process of rebuilding the rotation now, while also fortifying the chances of making the playoffs this Fall and being competitive in them. The Dodgers and Nationals will still be better but Quintana will give them a much better chance in a 5-7 game series, assuming they get there. The window is open now.