I can understand Jurhum's point, and in theory a caliphate sounds like a good idea.
In practice however I don't see how it could possibly work.
1) Does anyone honestly think a Persian and a Saudi, for example, could meld ceaselessly into this new "state"? Think how different these two countries are alone. Their culture, history, language and even religious practices are completely different. How could this problem be squared? Think East and West Pakistan in the 1960's. Different situation but useful as an example. All that held them together (as would in this situation) was religion, their language and culture was completely different. Soon enough East Pakistan (Bangladesh) got pissed off with their western rulers who understood little of their language and culture and won their independence. Religion was all they had to bind them. It simply wasn't enough.
2) Who would lead this new Caliphate? Moderate Sunnis? Wahhabis? Shias? A mixture of all? Even if an agreement was reached, would the mainstream be happy?
3) Finally, fortunately or unfortunately however you view it, in today's world there is a recognition that people are different. The Kurds are proud of their history and culture and thus want their own state. The same is true for the Persians and the Lebanese and the Egyptians etc. How many of these ancient people would willingly be subsumed into a gigantic Islamic Caliphate? These days, IMO, empires just don't work. Think of the amount of bureaucracy that would surely become involved in such a gigantic state. People want to rule themselves and feel a sense of pride in their homelands. Could this be maintained under a caliphate? I just cannot see it working.