It is interesting how quickly things can change
Back around 2012 or so, before Benghazi, Hillary's approval ratings were sky high. The traditional wisdom was that it didn't matter who the republicans ran against her, they'd have no chance. But Benghazi and all the republican inspired hearings (where were they for all the people Bush/Cheney left twisting in the wind in Iraq?) chipped away at her popularity. But her real damage was self-inflicted, using that personal email server and not coming clean and admitting it was stupid from the get go. She defended it so long by the time she did apologize for it, it read like the apology a guy about to be executed makes in his last words - too little and way too late.
Hillary was the only democrat Trump could have possibly beaten, because she was so disliked and inspired democrats so little they simply failed to turn out in anything like the numbers they did for Obama in 2008/2012. Likewise, Trump was the only candidate Hillary would have had a prayer of beating, even a religious nutjob like Ted Cruz who would never have a chance against a normal democrat would have beat Hillary easily without the election day drama Trump had.
Every once in a while the democrats seem to nominate someone who "deserves" the nomination as if it is some sort of lifetime service award, like Mondale or Hillary, and it bites them in the ass. The only time the republicans did that in recent history, with Bush Sr., he was lucky in his first try to run against a candidate who had all the charisma of a wet dish towel.
Republicans better hope Trump makes a better president than he does a campaigner, or they will be in for an epic landslide defeat in 2020. His negatives will only get bigger when he's in the public eye every day for the next four years, and since there is no heir apparent waiting in the wings to run in 2020, they aren't going to hand Trump another weak and widely disliked candidate on a silver platter like they did this time.