It is not a “natural retention” of “aggressive dog fighting nature” at all.
The traits being discussed over and over again here are being and have been bred deliberately into these dogs for hundreds of years. By dogfighters. For money.
It's not really either/or but both. The dogs that were initially bred for aggressiveness were the breeds that already had a high level of natural aggressiveness. If you want an aggressive dog breed, why start at ground zero? All the breeds commonly called pit bulls came from predecessor breeds that were already ferocious. I doubt you will find much cocker spaniel in their lineage.