Also, not that it’s the point but I have to note that technically most election victories are decisive, in the sense that they resolve the winner with little to no ambiguity (which is usually the case, even when the margin is narrow). In that sense, the only way Trump’s victory is not decisive is if you contest the legitimacy of the whole election.
This is such pedantry that you might as well say “the Merriam-Webster dictionary defines decisive as…”
I see calling like Raegan’s margin in 1984 “decisive”