Obama's legacy was always transitory. 

When what you regard as your major accomplishment is enacted on a party-line vote in the middle of the night - when even its strongest Congressional backers cannot explain what's in it - you can expect it to be overturned if the White House and Congress are suddenly in the hands of the opposing party.  When you promise people they will be able to keep their insurance and their doctor while saving $2500 per year and those promises turn out to be lies, your major accomplishment is likely to prove transitory.

When most of your other accomplishments are also done without Congress - in the form of government regulations and executive orders, you can also expect those to disappear like a puff of smoke when the political winds change direction.

Obama's biggest accomplishment was making US feel good - temporarily. 

Obama won the White House because Americans thought it would be cool for US to have a black President.  And it was.
For a while.