Trump is only still barely above 40% because there are a substantial number of Trump voters that don't like Trump, but voted for him so they want to "wait and see".
The moment that 10-15% I mentioned are negatively affected in a way that they KNOW is caused by Trump, they'll stop supporting him.
Trump has already disappointed a number of his voters; those who realize he will actually repeal the ACA and they'll be worse off, and those with family caught in the immigration confusion. Even a few who were probably turned off with him focusing on inauguration crowds rather than
anything else and congress trying to nerf the ethics committee.
I imagine the bulk are still quite happily in the "hey, look at all this action!" mode.
Once those "wait and see" voters (or their friends or family) lose their healthcare entirely, see their insurance premiums rise or terms change unfavourably, lose access to Planned Parenthood services, don't get their coal job back, see terrorist attacks happen anyway and the country remaining "unsafe", start paying more for Mexican produce, and/or are exposed to ongoing food/water poisoning, I certainly expect he'll lose more favorability. And that only accounts for what Trump has planned, much less whatever hare brained schemes he comes up with along the way (that Bannon feeds him).
Of course, recent history has proved an unfavorable candidate can still be considered "better than the other guy at least", so we'll have to wait and see what kind of candidate Democrats put out there in the face of poor favorables for Trump.