Interesting thoughts.

There was someone I was listening to recently, can't remember who, that suggested that agricultural societies eliminated hunter-gatherer societies over time mostly unintentionally through disease. Since many diseases jump to humans in close proximity to domesticated animals, herding societies would encounter hunter-gatherer and the hunter-gatherers would die from some flu they had no immunity for. It is not thought that herding existed 40K years ago when neanderthals went extinct (or were assimilated) but I think similar hypotheses could have some merit.

So, by your hypothesis, neanderthals and denisovans left Africa earlier and were assimilated/absorbed into populations that left Africa later. It's interesting.