If anything, the presence of 'other scenes' has had an inflationary effect on the rare soul scene. Take a record like Mixed Emotions 'Gold of My LIfe' formally a cheapie at the Mecca. But thirty years have increased its scarcity, the modern northern scene has increased its value, the 'B' side is very collectable funk track, which has pushed its value further. So in effect, more people have a reason to collect it - but supply has not increased. So if demand increases and supply stays the same, then just like oil, the prices rise. That's basic economics.
The emergence of the gay disco scene has made Philly and NY Disco more collectable, the rare groove scene has made JB and Street Funk more collectable, the rare funk scene has made Ohio labels the 'new Detroit' , the R&B scene has its effect on prices too. The Japaneses collectors market has made small southern based labels much more expensive than they were in the days of Wigan, where they never really 'competed' with Detroit and Chicago releases.
We often assume the inflation in the market is about the rare northern scene and 'people paying daft prices' but there are probably more people in Japan bidding for records like Don Varner 'Tearstained Face' now than there are northern collectors. There are definitely more US buyers for indie funk.
Even the evolution of our own scene has played its part in the demand curve. A great current example is Ollie Jackson. Old school collectors around in the '70s would not part with ' The Day My Heart Stood Still' despite the more recent demand for the flip side 'Gotta Wipe Away the Teardrops' . So if copies are scarce, and there is a new demand surge then the price goes up.
....and there is more to come, the two biggest populations in the world the Indians and the Chinese have yet to catch the soul collecting bug...
Great.