It's an emerging country. It's gigantic. It's more than twice Afghanistan population, but with actual production facilities. They are richer than Irak was, but they have mountains.
Also, Ismail Khan is likely in Iran, but still have his power base in Afghanistan. Likely, Taliban warriors won't care about ideological differences and go fight on the Iran side even without him.
You have to see Iran like an ex-USSR nation. You do not want to invade. It would be too costly. At least Vietnam War level.