It doesn't matter how much it has invested in Syria. Every fukking Sunni is looking at what's happening at Aleppo and all they see is Russia helping the Syrian regime slaughter their people. The game is different.
I'm not defending Assad's crimes and the Russians bombing of areas where they've killed civilians. I believe Russian strikes have killed thousands since they entered the war last year. There will be blowback but the Russians aren't going to care as long as their objectives are met.

Also...
The sectarian narrative has become entrenched but the facts on the ground aren't as black and white.
Yes, Iran and Hezbollah are playing their roles. Iraq has Shia militias there too. But this war isn't Alawi/Shia vs. Sunni.
It's more like Sunni/Alawi/Shia/Christians vs. an opposition of Sunni Syrians from the rural areas who were forgotten for years and subjected to authoritarian governance along with opportunistic foreigners and the psychos Assad released from prison.
Majority of the Syrian Army soldiers are Sunnis. The majority of residents in Aleppo are Sunni (Western Aleppo was under government control for most of this war and it has 1M people). The Sunni upper and middle business class in Aleppo and Damascus has always supported Assad which is why he's still there. Pro Sunni govt militias have been assisting the SAA including Palestinian groups.
The dynamics of the war started with the Sunni population that lived in rural areas of the provinces and were of poorer or lower middle class. That's the population that rose up to demand better economic livelihood and political representation.
Unfortunately, the radical groups have proven to be the best fighter and Syria has become the flash point to recruit radical Muslims to fight. Thanks to KSA and Qatar... along with other wealthy gulf donors