Maybe it is better as another thread, but.. there is a related topic that is neglegated in such threads, I discovered - the holy languages, or traditional languages of use in religions.
Alternate religions, alternate 'holy language'... would a form of persian take the position of latin by example, if europe choose zoroastrism? taoism over south asia, classical chinese a common scholar language?
I think that's an interesting question. If the religion has a holy language then its probably the language of the creator of the religion (as in Islam with Arabic) or the ligua franca of the most powerful country that it becomes dominant in (like Latin in Catholicism). So Latin, Greek, Persian or Mandarin Chinese would probably still be the most important religious languages for lots of religions. If there was no Islam, Arabic would probably not become important (assuming that Arabia does produce another global relgion and Arabia doesn't produced any empire, which it hasn't beyond the original Caliphate)
Monotheism has the advantage of being more easily centralised. Of course all religions are subject to ideological splits or power struggles but with Polytheistic religions its that much easier because you just create a new God or a cult to an already existing God. Monotheism is predisposed to a hierarchy and organised institution, Polytheism is more susceptible to syncretism or even assimilation. If the Gods themselves are at odds chances are their religious followers will be too.
Monotheism is inevitable in the sense that religion is inevitable, stories of heroes quickly become legend, legends are embellished with myth, and over time these myths are accepted as truth. Once you have your Pantheon, unless there's a greater theme of harmony (as often found in creation myths) then yes, the Gods have feuds and wars and one or other will rise to prominence. Sometimes this is even dictated by the social standing of that God's following here on earth, ie between babylonian city-states for example. Henotheism is not necessarily doomed, but it doesn't allow for religious fanaticism in the same way.
Monotheism is in no way inevitable, and most pagan religions have a main god of sorts. Egypt had Amon Ra, Rome had Jupiter (actually in his own trinity as well for a bonus!) and Hinduism has Brahma (again in a trilogy ). The irony with Christianity is that it became like pagan religions to appeal to a wider audience, hence becoming more polytheistic. Unlike Islam or Judaism there are three gods (combined as one), Jesus (who is basically treated this way in most branches of Christianity too) is generally treated as a separate and equal entity, so is the Virgin Mary and a million saints. Lastly, its the largest branch of Christianity and Hinduism is the third largest religion. None of this say monotheism is inevitable to me.