The Volcanic Winter was world-wide, significantly effect crop production - famine. Justinian’s Plague was particularly hard in the east. Both effected populations, hence effected trade, economies and the ability of the Eastern Empire & Persians to maintain and raise troops. As you pointed out both empires had devastated themselves with a 40 year war, the pressures of the plague and Volcanic Winter left a scarcity of resources to rebuild. They became a hollow shell. The Arabs essentially punched an empty bag!
You know, I did not know about that volcanic winter - or i may have read it somewhere and not focused on it, but your points are very very good.
Thank you! you’ve taught me something new today and I am grateful!