If they can't win in 2018, and Reps will make gains as you think, I don't see how you see any chance of 2020 being that different. The Senate map will be much better, but the hurdle the House faces that you deem as impossible to clear will still be there. If it's impossible in 2018, it's probably just as impossible at any point in the future. And if they make gains in the Senate, it'll probably negate any progress Dems manage to make in 2020 back to a slim GOP majority
Democrats, particularly young democrats, will actually turn out in 2020. That's the difference. If we could get people to vote in 2018, perhaps it could be different and the Republicans actually sitting in districts
that Hillary Clinton won might have a chance of losing. But I don't hold much hope in that actually happening. Too many of my generation and younger can't be bothered.
Get the 18-29 and 30-44 groups to turn out in 2018 and surprise me.
When those lines are falling 20% or more in midterms and the top lines are dropping closer to 10%, and with older voters skewing Republican, Democrats are at a decided disadvantage in midterms. Can they overcome that? Sometimes. But if you add that disadvantage to a gerrymandered map, it's not happening in the House. And Democrats will have their hands full defending their own seats in the Senate.
2018 is toast. When young voters vote again, it will be 2020, and vulnerable Republicans can go down.