I remain skeptical of the oft-asserted claim that "monotheism wins, polytheism fails" - religions are generally always in a state of evolution and flux. Christianity filled a void in the social and religious world of late antiquity, but that void could have been filled by any number of things. In the absence of Christianity - say a no Jesus or a less successful Jesus PoD, the "winner" of Rome's heart and soul won't just be the next monotheistic faith to show up but rather the first faith that manages to successfully make religious experience more personal and more individually meaningful.
Still, no reason there has to be one winner. I rather like the idea of a blur of competing cults and divinities. Isis and Serapis remain a typically under-considered duo, to say nothing of the fact that certain late Hellenistic ideas of god rather sound like the kinds of things that certain Hindu schools might develop. I wonder if we couldn't have a world where all Gods were aspects of a single prime divinity.
The key was to move past ritual and the regulated order of late antiquity paganism into something more personal and more moving. I think paganism could do that and indeed there was every indication it was moving in the right direction before Christianity superseded it. In uncertain times people want to feel safe. People like feeling like they're part of a big cosmic battle or some sort of struggle that's bigger than themselves. If a nonchristian faith can provide these things, then Christianity becomes less necessary and indeed less likely to triumph.
