When you talk about the current 'main hypothesis' I assume you refer to the popular consensus theory? The leading academics are currently starting to invoke multi-regionalism, but limited to regions of Africa.
We have a major flag for interbreeding between a Denisovan population (Southern Denisovans) and ancestral Aboriginals in Oceania. The traces of Denisovan genes elsewhere are at trace levels or nil.
I can understand why you would think that this makes an Out of Australia migration an unlikely explanation for the populating of the world beyond. Surely everybody should carry the same levels of Denisovan genes were this the case?
The answer comes in the dates. The populating of Eurasia began around 60,000 years ago and was well underway 55,000 years ago. The interbreeding between Aboriginals and Denisovans occurred 44,000 years ago, explaining why we do not see this genetic material carried outwards, the meeting occurred after the migrants had left. It also means it can have occurred only one place, on the Australasian continent.
We know today that Africans also carry Neanderthal DNA, not at the same levels but it was brought into the continent by migrations of modern humans a number of times. I do not know the scale of the migration into Africa 73,000 years ago, but it likely did carry Neanderthal genes into the continent and I would expect Africans with the haplogroup L3 to have Neanderthals genes above trace levels. The migration into Africa 45,000 years ago seems to have been fairly limited to the North of the continent and the Levant-Egypt region. Africa had it's own resident population so genes would have been 'watered down' or been kept back by any barriers to interbreeding (such as strict marriage lore). I expect Neanderthals genes are found at a higher percentage in North Africa than other regions. Might not be, I have not checked.
Thanks for your comment.