Republicans are going to get demolished in the midterm elections. The reason they were put into power in the first place was to repeal Obamacare. The fact that congress has been so impotent in doing anything on healthcare will ultimately lead to their demise. Trump still has a chance in 2020, but that hinges on the economy and a southern boarder wall.
You are correct that the Republicans will not do as well as they should given their built-in advantage with gerrymandered constituencies but not for the reason you say. Most voters would want to keep the Affordable Care Act and it must be doubted that they voted Republican because they wanted to be dropped from health coverage. Rather, the reason Democrats will get support is because they look more efficient and caring in government and Americans are generally embarrassed with Donald Trump.