Most statewide offices may be decided in 2018, but the legislatures are not immune from whatever happens in 2020. Incumbent governors tend to win reelection though. Only Ehrlich went down in 2006 and Strickland and Culver in 2010. If a Democratic candidate wins in 2016. there's nothing to say that the first midterm will necessarily be disastrous like it was with Clinton or Obama.
There are many state legislatures that were drawn by Republicans to be unwinnable for Democrats, while a state itself cant be gerrymandered. Even if 2020 is a huge Dem wave, the Ohio legislature isnt flipping, so having a Dem governor elected in 2018 would allow them to be there in 2021 to veto a GOP gerrymander and force fair maps. Same is the case in Pennsylvania and Michigan. Democrats getting fair maps in those states alone would probably net them 12 or more House seats.