You missed the point. Obamacare and the Iran deal may have value but they have no sustainability without congressional support. Trump is forcing Congress to act because without Congress presidential authority has no power in law once the presidential term is over. Thus the vulnerability of Obama,s legacy. If Trump gets Congress to pass legislature the legacy will be lasting. I might add that if  Congress fails to act any presidential decrees Trump uses will pass with his term and he will suffer the same fate as the Obama legacy.