Sanctions did not bring Iran to its knees.
The war in Syria busted Iran. Iran like so many countries in the West found out how war busts you financially. It was the endless bottomless flow of money to support Syria that was the end game financially for Iran. They were still selling oil despite sanctions, notably to India. But their Syria support bled them.
Now Iran has the money it needs to support Syria financially, to afford to keep sending weapons and ammunition, to support the astronomical costs of thousands of Hezbollah fighting ion Syria. And to finance the supply of military hardware flowing through the tunnels into Gaza and the continuing fortification of Southern Lebanon.