It's a very interesting point. A lot of the American economy is performing strongly. Unemployment is low, results are good. The question is whether this is because of Trump or despite of him, I guess, and that is one I sort of feel I have to leave to the American people to decide.
One thing I would note is that the current shut-down has occurred when the Republicans have a president and also a majority in both houses. That suggests that his support within the party may not be that strong.
I also think that he may get a right $#@!ting in the mid-terms. The anti-Trump vote will be motivated to be out. And that could be why those house majorities are not staying loyal to him.
If he gets through the mid-terms without too much damage below the waterline, and nothing else demonstrates his colossal lack of competence, then he could still be the Republican candidate at the next election. Don't expect the Democrats to put up Hillary against him though. They will surely have learned from that.