Why arming Syria's rebels wouldn't have stopped ISIS

"An insightful essay by George Washington University's Marc Lynch explains why."
You follow this down the rabbit hole, and you get to the claim that even John McCain thinks arming the rebels wouldn't work.
'There’s no way to know for sure what would have happened had the United States offered more support to Syrian rebels in the summer of 2012, of course. But there are pretty strong reasons for doubting that it would have been decisive. Even Sen. John McCain was pretty clear about this at the time, arguing that arming the rebels “alone will not be decisive” and that providing weapons in the absence of safe areas protected by U.S. airpower “may even just prolong [the conflict].” '
And then you read what he says, and he thinks arming the rebels will do tremendous amount of good, it might not beat Assad completely, but still it's all good. And the implication that American machinations are more important than the actual war Assad is fighting does not really qualify as insight either.
“First, the fact that the opposition in Syria is doing better militarily thanks to external support seems to validate what many of us have been arguing for months – that opposition forces have enough organization to be supportable, and that our support can help them to further improve their organization and command and control. This is an argument for doing more, not less, to aid rebel fighters in Syria.
Second, while it is good that some foreign military assistance now seems to be reaching the opposition in Syria, this alone will not be decisive. It will not be sufficient to end the conflict faster. It may even just prolong it. Nearly every Syrian I speak with tells me the same thing: The longer this conflict drags on, the more radicalized it becomes, and the more it turns into a sectarian civil war with an escalating spiral of violence that Syrians alone cannot stop.
Finally, the Syrian opposition needs to know that the United States stands with them, and that we are willing to take risks to support them when they need it most. Our current inaction only denies us the opportunity to have influence with the forces in Syria who will one day inherit the country. And we are ceding that influence to foreign states that may not always share our interests and our values – or worse, to extremist groups that are hostile to us. Our lack of involvement in Syria is not preventing the militarization of the conflict, or lessening the risk of sectarian violence, or countering the appeal of extremist groups. All of these events are just happening without us – and without our ability to influence them.”
[http://www.mccain.senate.gov/public/index.cfm/speeches…]