This is quickly becoming the narrative now, that Russia and Iran just said "meh", and chose not to support #Assad in his time of need because they didn't see sufficient value in it.
I personally very much disagree with this.
Iran has invested significantly in Syria, and was looking to leverage multiple economic projects across Syria to recoup some of its "investments". At the time of Assad's fall, it was still sending tankers loaded with crude oil, who had to turn back once the regime fell. It still had militias inside Sayyeda Zeinab. Hezbollah sent troops to Homs and Qusayr.
Russia was still striking opposition fighters, in the hours that preceded the fall. There are claims that some Russian soldiers are stranded because of how fast things went. For Russia this is potentially a massive loss, that puts into question how they will continue to project influence outside of their immediate vicinity, including in Africa, the Mediterranean and to a lesser extent Libya. The Russian "life insurance for dictators" has shown itself to be empty.
I think there is a much simpler explanation: Given how fast things went (less than two weeks in a conflict that last more than 10 years!), Russia and Iran were completely stunned, and unable to decide and quickly sent enough reinforcements. The regime was hollowed out by its own corruption, and the fallout of continued economic crises (including in Lebanon). Reconciled rebels switched side. Former rebel groups re-emerged. The whole thing was a dried out tree branch waiting to catch fire.
Russia and Iran (particularly Iran) seemed to have rested their hopes on diplomacy (I know, quite unusual, but that's what they had). They hoped contacts through the Astana format would help stop or delay the offensive. That the diplomatic clock would stop the military clock. It didn't, and there you are.
But to say they just looked at Assad and said "no thank you" - I don't believe it.
Don't believe Russo-Persian fairy-tales about abandoning Assad. They tried to keep him in power but still lost.