The pain here isn’t so much at Senate and House level....it’s at the governor and state-level that the Democrats need to worry about. Census episode will occur in 2020, and these will be the folks drawing the redistribution of house seats. At least four states are figured to lose one seat, and at least four states are figured to gain. All four of the gain (likely) will end up being GOP-gained districts.
The other reason of course is their farm bench got decimated in the last several election cycles.
They can’t attract and recruit talent because no one wants to serve decades in the minority.
Ambitious people will run as Republicans.
They’re simply screwed.