I understand what put him over the top--Comey re-opening the email investigation two weeks before the election only to find nothing; voter suppression by GOP state govs in Wisconsin, Michigan, Pennsylvania; and the social media activities of the Russian government's Internet Research Agency. But it shouldn't have been that close in the first place--Hillary had no charisma and too much baggage, even if it was overblown.
But even without the Russia connections, I think Trump's terrible policies will catch up to him--the giveaway-to-the-rich tax bill and the Chinese trade war chief among them.
Sources:
New York Magazine - Voter Suppression May Have Won Wisconsin for Trump
Washington Post - The Dutch were a secret U.S. ally in war against Russian hackers, local media reveal
WaPo again - Timeline: How Russian trolls allegedly tried to throw the 2016 election to Trump