Eh, both Carter and Reagan had positive favorability ratings. Clinton and Trump are both very high. I know quite a few people voting Johnson this year in roughly even numbers from Rs and Ds based on registration/2012 vote. Now if wikileaks or Project Veritas drops a monster bomb a week before the vote (or something against Trump), you are probably right but otherwise I think 3rd party will get a lot more votes this time than normal.
Sample size of 1 (one) -- I don't "approve" of Trump and I self-identify as a "small-L" libertarian. But I'm sure not going to throw my vote away on Johnson against Hillary.
We'll see, "in the fullness of time".