With Libertarian candidate stumbling on Aleppo in an interview a while back, and the polling evidence that his support is pulling voters more from Hillary Clinton than from Donald Trump, it was only a matter of time before the liberal media piled on tried to suck all of his support away from him as quickly as possible. With Clinton having, at a minimum pre-debate, having lost momentum, it's become necessary to find her a path to victory.
Trump did not tank at the debate, even if the perception is that Hillary might have won the debate, she did not sink Trump. Trump did not sink Trump either. So the Gary Johnson thing might really matter.
It's no coincidence that a progressive liberal (socialist) leaning network like MSNBC is trying to give Gary Johnson some rope to hang himself by having him on. Bernie Sanders supporters are generally younger and certainly feeling the sting of being treated as outsiders after the scandalous DNC attempts to ensure a Clinton victory in the Democratic primaries. And Libertarian candidates like Johnson have tended to have more success among the same demographic than older ones - at least when it comes to attracting new voters.
So the MSNBC strategy is targeting correctly - get those voters who you can possibly peel away from Johnson and get him to slip up again. Clearly Johnson is an isolationist and another slip up on foreign policy was/is bordering on inevitable.
Gary Johnson had an "Aleppo moment" after @hardballchris asks who his favorite foreign leader is #JohnsonTownhall https://t.co/nRazpPL0q0— MSNBC (@MSNBC) September 28, 2016
Whether it just turns them off from voting this cycle entirely rather than turning them to Clinton is another matter. But you won't convince me that this was not a deliberate vote grabbing attempt by MSNBC.