"Why do you believe DC statehood to be an impossibility if Democrats get the trifecta in Congress and the Presidency?"
Because they would need to amend the Constitution, at the very least to eliminate the three EVs given to DC. Remember, there would still be a "district of government" comprising the White House, Capitol, Supreme Court, etc., and it would be untenable for it to still have 3 EVs after it is stripped of all inhabitants (except arguably the President and his family). This would also be necessary in the case of the expanded New Columbia or if DC was returned to Maryland, but GOP opposition to those proposals would be far less.
The Democrats had supermajorities in Congress from 1965-1969, along with LBJ in the White House, and DC statehood went nowhere. They also controlled both houses and the White House from 1977-1981 and from 1993-1995, with the same result. I think DC residents should seek a compromise measure, such as a larger New Columbia.