According to previous elections, this is indeed true. Third-party candidates tend to lose support closer to the election dates. (graph below)
Usually, third-party candidates will hold onto their support into late August according to FiveThirtyEight.
Why is support for third-party candidates dropping?
There are a few reasons for this --
First and foremost, voters will start to make up their minds closer to the election date. One notable example is Ross Perot, who ran as an independent, after the Republican and Democratic conventions, who is widely considered to be the most successfully third-party candidate in modern days. Party conventions remind regular voters whether they are a Republican or a Democrat, increasing their support for major party candidates.
Secondly, some voters will realize that the third-party candidate they are supporting won't win even if they vote for them. Wouldn't it be a better idea to vote for Clinton to stop Trump or vice versa. At least, they cast a vote to stop their disliked candidate from getting into the White House (this is known in political theory as "tactical voting", and is one of the many reasons people consider FPTP electoral systems inferior to those like IRV).
Thirdly, you mentioned that third-party candidates might not have the intellect to be president, given the Aleppo moment and unable to mention a favorite world leader. After sometime, voters will realize this.
As to why Gary Johnson is polling below 10%, the fact is that he simply hasn't crossed the 10%+ threshold. The New York Times' average polling for Johnson constantly polls him below 10%, at around 8% to 9%. The FiveThirtyEight forecast model (Now-cast) has him at around 9% to 10%.
Both Johnson and Stein didn't have a real chance for the presidency from the very start, since their combined popular vote doesn't even exceed 20%. To win the presidency, one would require at the very least 30% of the popular vote.
Why is there lesser news coverage for third-party candidates?
This is because not many people think that Johnson, with 10% of the vote, will actually win, so does news organisations. Since they aren't going to win anyway, the news won't spend the time mentioning them.
Also, they don't have as much events as Hillary Clinton or Donald Trump; they aren't invited to the debates; they don't have a lot of rallies, there just isn't much things for news to mention.
If there are, news organisations will report them, for example McMullin was mentioned after a few good polls in Utah came out recently.
To sum up, the drop in support for third-party candidates is caused by all the reasons mentioned above. Even though both Hillary Clinton and Donald Trump are disliked, many people votes for the party, not for the candidate which explains why people still votes for Trump.
This is also how safe-Democratic and safe-Republican states come about. Some voters support the party, not necessarily the nominee.
Below is a graph that shows the level of support for past third-party candidates: