I don't know how long a presidential legacy is expected to last. More than two hundred years (George Washington); more than one hundred fifty years (Abraham Lincoln); more than seventy years (FDR); more than fifty years (JFK); forty years (Reagan)?
Until 9:00 p.m. November 8, 2016, I assume a lot of folks thought President Obama's legacy would last a decade, maybe more. First African-American president (not counting Bill Clinton); eponymous massive healthcare bill that everyone loves (except those who can't afford it); and, some other things.
Then between 9:00 p.m. and 3:00 a.m., Tuesday night/Wednesday morning, the duration of that legacy began to shorten.
Eight years? Five years? One year?
I'm not sure his legacy will last through the end of next week based on the reaction of the stock market.
The day after the election, the election in which there were no margins so close that a recount was needed (Michigan is still counting), the stock market surged to an all-time high.
Two days later, expecting a bit of profit-taking, the stock market surged to another all-time high. And these weren't baby steps. These were two successive days of three-digit jumps.
The Trump daily thought: Obama's legacy could be erased before he's even out of office; the legacy could be erased by the end of next week.