Looking solely at 2017, the Cubs can certainly win the division and get to the postseason with Quintana's help. That alone beats the alternative of missing the playoffs entirely. Once in the tournament, anything can happen. The cubs just need to get hot at the right time while Washington does its typical October swoon and the Dodger pitchers fail to duplicate their regular season dominance.
In the longer term, there is a need to replace Lester and probably Arrieta as well.