I’m not sure which is more pathetic, the misconception that Trump can destroy Obama’s “legacy” or the fact that so many of his followers don’t understand what a legacy is. A legacy is what you hand over to someone after you. What they then do with it is then their legacy to hand over to someone else.
History measures Presidents by their actions and accomplishments (and inactions and failure) while they’re in office. What happens after they leave office then becomes a measure of the next person to hold the office. So, for one of many many examples, history will record Obama’s brilliant end-run around a racist/obstructionist Congress to make an historic accord with Iran (and several world powers) to prevent it from achieving nuclear status, as one of many remarkable achievements.
When it comes to Trump, however, if history records anything other than “First President To Accidentally Tweet A Picture of Him Fucking His Daughter,” at best it would be something along the lines of, “For no logical or discernible reason other than either jealousy or a pathetic appeal to white bigotry, Disgraced President Trump reneged on the deal with Iran, which in turn led directly to Iran achieving nuclear status within a year.”