Russia and allies doesn't mix.
Russian foreign policy is self-serving. No more Soviet style friendships.
If You speak Russian You will notice that Russian foreign policy calls other countries PARTNERS. Yes, not allies, enemies, but PARTNERS. Even US. I think Russian foreign policy have matured (domestic one is still shit tho).
Russia won't take bullet for You, sooner You guys understand that the better. I think Russia had hard lesson with Ukraine about it.
It's better Iran than US boys in Syria.
Plus it will hard for Iran to actually copy it as it requires not only know how, but also logistic and technical background. If You buy new BMW it doesn't mean You can copy it and produce it by Your own.
Not saying that Iranians are bad, but even Chinese are having troubles when it comes to cruise missiles. Russia is no 1 in the world when it comes to this.