IMO, had the U.S. increased support to Syria's sectarian elements, one might actually be dealing with a stronger ISIS, not a weaker one. In that case, the current dictatorship might have been sufficiently weakened to permit ISIS to take control of the entire country.
On the point about building a coalition, Reuters reported:
U.S. Secretary of State John Kerry pressed Arab leaders on Thursday to back President Barack Obama's new military drive against Islamic State, calling for tighter curbs on funding for militants and fewer extremist messages in Arab media.
Meeting Arab leaders in the Saudi city of Jeddah a day after Obama announced his plans to strike fighters in Iraq and Syria, Kerry also sought permission to make more use of bases in the region and fly more warplanes overhead.
Kerry presses Arabs to back campaign against Islamic State | Reuters
IMO, these are things the President should have done before he finalized a strategy and before he gave his speech. In effect, he's asking the Arab leaders to accept a strategy in which they had little input and may not necessarily address their critical interests. Basic approach: Establish goals (with input), align support, then announce. What happened is that the President set a strategy before he aligned support, and then announced the strategy proclaiming a broad coalition which, in fact, does not yet exist.
Finally, Russia has now weighed in. The same Reuters piece reported:
The prospect of U.S. armed action in Syria also drew concern from Russia, which has backed Assad. In Moscow, the Foreign Ministry said air strikes in Syria would require a U.N. Security Council mandate or be considered an act of aggression, Interfax news agency reported.
Given the precedent in Libya and announced increase in arms to sectarian groups in Syria, concern by Russia among other pro-Assad countries that there is a "backdoor regime change" component is not unreasonable, especially as no controls or mechanisms to preclude that outcome were mentioned. Considering that Arab support was not lined up in advance of the speech, my guess is that the U.S. never considered the possibility that Russia, Iran, and other Assad backers might have concerns and might also take measures of their own.