An investigation by Elizabeth Tsurkov entitled "The Breaking of Syria's Rebellion" has elicited debate over what broke it. Did it break because the rebels were corrupted, authoritarian, feuding, oppressive, or did the rebels get that way because something else broke it - most likely Russian and Iranian intervention. It's not the sort of debate that can be definitively resolved. But there are reasons to think the breaking came first.
The interviews on which the study is based took place since, it seems, about mid-2016. Supposing by then that the revolution was broken, the findings diminish in significance. Of course if things seeemed hopeless, corruption would set in; discipline would break down; authoritarianism - a given when an area is under severe military threat - would intensify, and the mass of people would resent being exposed, for nothing, to the horrors of war. And of course the most determinedly ideological forces, in this case radical Islamist militants, would be seen as the lone inheritors of the struggle. But when was hope lost?
In my opinion hope was lost well before a full-fledged Iranian/Russian intervention. It was lost roughly when Obama erased his own red line, about mid-2013. About the time of his reversal, he also made it crystal clear, despite reports to the contrary by Michael Weiss and other supposedly knowlegable journalists, that the US would never seriously supply the rebels with what they needed to counter Assad. (I argued this in some detail in June, 2013.) In particular, that meant the rebels would never have the capacity even to moderate the régime's air attacks. Indeed, the US did not merely refrain from supplying the most minimal anti-aircraft capacity; it forbade other parties from supplying such weapons as well. This inability didn't necessarily mean the rebels would be wiped out, but it did mean they couldn't win. It did mean that civilians were being exposed to the barrel bombs and other attacks for - it was becoming clear - nothing.
So it was the US betrayal that really marked the turning point. Its effects were delayed for some time because the US took another year or two entirely to abandon the pretense of supporting the rebels against Assad, rather than as a mere proxy force against ISIS. But past that point, if hope did not die instantly, it died slowly. The demoralisation of the rebels and the degradation of their cause - so carefully documented by Tsurkov - was then inevitable.