Consider if the GOP loses the House. First item of business will be impeachment of Trump which will breeze through the House on a party line vote. The impeachment trial in the Senate will drag on for months with jackals in the media salivating at the prospect of finally bringing down Trump. However, like Clinton, Trump will survive the impeachment and go onto win convincingly in 2020. The Democrats will have alienated at least half of the population, no legislative agenda will have been accomplished and Trump is not only still in the White House but elected for another four years. If they take the House, the Democrats doom themselves with impeachment, but have no other agenda other than raising taxes and opening the borders... both non starters with the electorate. Their election victory in the midterms will gain them nothing.
I respectfully disagree. First order of business is the election of the speaker. If the Dems have a slim majority (less than 5 seats), then the wrangling over the speaker will actually be the biggest issue in the House, not impeachment. After that is settled, the incoming speaker will be too wounded to take up impeachment.
In other words, impeachment will only happen if the Dems win a comfortable majority (more than 5). Impeachment will be a non-starter if they win the House by 1 or 2 seats (possible) because of the amount of horse trading that will be needed to keep everyone in line.
In short:
Dems win the House by 10+ seats (net gain 30+): Impeachment is certain and will be attempted
Dems win the House by 5-10 seats (net gain <30): Impeachment is possible and may be attempted
Dems win the House by fewer than 5 seats (net gain around 25): Impeachment is highly unlikely due to prevailing dynamics and will not be attempted