Let's see if you can think it through.
Turkey is a member of NATO and historically a major ally of the USA.
Russia is a major supplier of arms to Syria.
Iran is a major supplier of arms and troops to Syria.
Russia and Iran both border Turkey.
Remember, WWI started because some penny anty duke got shot.
There's a downside to every conflict and that is innocent people get killed. While you are correct about a minor incident creating a cascade of unintended consequences the alliances and the Grand Games being played did indicate a high probability of a small match causing a great catastrophe.
The times and circumstances are different now. The chief contributing factor in WWI was the inability to disseminate information quickly. In other words while in retrospect it was easy to see cause and effect, what could be known by any power was limited. From their perspectives they knew of potential problems but had only supposition. Contrast to what you've said. We know who would support Syria and by what means. While you are correct, there are known considerations to examine. First, the Russians will offer limited help to Syria, but not because they back them. It's part of that Grand Game which powers use others as surrogates against the other great powers. It's been going on in royal courts for thousands of years, and extended to nations when possible. It's about power. Control. This is what drives such things, not mere money. The ultimate dream is to do what Thulsa Doom did in front of Conan, when he called a subject to her death off a cliff by the wave of a hand. That's Power.
Russia however knows that it's ability to exert it's influence is limited by others and it's own interests. It may support Iran for the same purposes it does Syria, but it does not trust it. The calculation is that they must work with the P5+1 at times even as it works against it. That is the Game- the challenge of balance.
Bottom line, Syria may benefit but not at the risk of Russias other interests. Support will be limited.
Then there is Iran. Their leadership backs Syria because it is a thorn in everyone's side. They engage at a less subtle level in the Game because they must. If they acquire power such as nuclear weapons their influence will be less direct and more expansive, but that is not today. Now they act with support, but they have limited capacity.
The result is that you are correct in that Syria will get some help, but what is needed is not munitions, but trained men. That they do not have, and with the West solidified in their condemnation of Syria no one will provide that. In short Syria is screwed and there isn't a list of players willing to invoke losses on their side to create a global disaster.