To be clear, I don't think that "monotheism" is uniquely positioned to defeat "polytheism" throughout the entire history of the world. That's nonsense, as East Asia+India demonstrate. As do other regions and times in history. At most I think that in the world of the Roman Empire, in the 3rd to 4th centuries the trend seems to be that a monotheist religion was heading toward triumph, or at minimum a pseudo-monotheist religion where one supreme deity was above all others. Ie, someone like Sol Invictus.
I would be very interested to see an argument to support monotheism already being on track for triumph
in the 3rd century. Not having seen such a thing, I highly doubt it. No new system was poised for any meaningful replacement pre-Constantine, according to any historian I've encountered.
As to "pseudo-monotheism," there's no
there there. Even the big Sol Invictus guy Aurelian cheerfully put priests of the deity as peers to other pontifexes. He allowed many priests of his new major deity to
remain primarily priests of other gods and only secondarily focus on the cult of Sol Invictus. And that's beside the point that I've not seen the slightest sign that Aurelian's enthusiasm might entail a rejection of
any part of Rome's existing religious practice.
So what would a long-term dynastic press for Sol Invictus look like? Just a new combination of chief gods of the Roman state, something that had happened several times before. All of the gods we are aware of would probably continue to be worshipped over the next couple centuries. A thousand years later some would have dropped from the picture or become significantly less important, as that did tend to happen. But unless something dramatic changed, the whole polytheistic system would remain in place, the Mediterranean would be very similar to modern India, and Sol Invictus would not be particularly special. He might be around, or might be the first of many gods to be temporarily elevated by a particular emperor, only to descend to obscurity.
There was no social or political mechanism to either invest Sol Invictus with permanent relevance or to induce a purge of "lesser deities."
No reason I can see why that would come to look much like a form of monotheism.
The thing about Julian is...he didn't actually want "traditional" Roman religion. He wanted his version of the Roman religion, which was both extremely different from what had existed, and also borrowed a lot from Christianity to replace both. So you ended up with a religion no one followed. Except him. There's the infamous story of him trying to sacrifice an animal before going on campaign and just...not finding anyone who cooperate.
Yeah, and I've said elsewhere that Julian was not just too little, but very close to too late.
But your argument partly misses the point. You could find people propitiating brownies in corners of 19th century Britain and France. That sort of thing has direct continuity with religious rituals to the little gods of Rome and its neighbors. Certainly well after Julian you had the Bishop of Milan complaining that Italy was full of nominal Christians who would run around gleefully indulging in the more popular pagan festivities. There were many people who hadn't stopped many rituals or even polytheistic worship in Julian's day, and in a hypothetical scenario where super-Julian is ascendent, many of those people are going to keep doing what they were doing. Julian doesn't need to actually reinstate the old order to enable dramatically more continuity with what remains of it.
Personally I don't think that quite avoids the outcome we saw, because there will be another Christian emperor before long, and in a unified Rome this would resume the squeeze. There were too many Christians and they weren't open to sharing the public space unless they were out of power.
But in a Julian-like scenario that came earlier? Sure. Traditional Roman religion hadn't been set in stone before - it had been in constant flux. Gradual change with occasional big shifts in the set of major gods - that was the default mode of Roman religious history. If Christianity fails to get critical mass (40% to pull a random number from the air), a Julian figure making up and funding rituals would mostly reset the Mediterranean system if they lasted, because the ambitious and upper classes would follow the politics and the money, and most people wouldn't even register which rituals great grandpa attended that were gone now.
