On what basis do you believe that a fanatic in a cave who probably doesn't even control one country can take over the world much less command a "vast Sunni caliphate." Let me point again that EVERY attempt to unite either the Arab, much less than Islamic world, have failed miserably.
Mohammed started by controlling one desert oasis. He is now established among peoples who inhabit a region that includes southern Afghanistan and northern Pakistan. His brand of Islam is the glue that holds the Taliban tribes together. Potentially he can gain control of all Pakistan, a country with nukes. As for Iran, there is nothing to keep them from supplying tactical support for the tribes to the east, so long as they can cause mischief for us. If we withdraw, the Taliban will again be at war with the Northern Alliance, and this time they may win. They could then extend their influence into central Asia, including western China. Iran will have been freed of a presence on its eastern frontier and can now concentrate on the Gulf States and Iraq—where they have had influence for years, both among the Shia and the Kurds. No telling how events will shape all this. The emotion may play out; important leaders may die. New leaders may emerge. Whatever, this is not a small, isolated event, if for nothing more than it will all be happening on the borders of some very important places, including India and China.