I would bet money that if the election was held again today that Clinton would win. Trump is doing some things that his supporters want, but he's also completely flipped on some of his promises (remember "drain the swamp"?) and pissed off a lot of people in the middle who voted for him. If his policies are home runs then he might survive, but I very much doubt they will be, moderate successes are possible, but they will likely come with serious downsides that will leave him open to attack quite easily.
Only time will tell, but I wouldn't put any money on Trump being the Republican nominee in 2020.