Harry:
And it least for the most part its is still a free Country here. I expect there are thousands of owners of the 458 Winchester Magnum that would totally disagree with your assertion.
Current ammunition "shortages" aside; anyone can walk into any well stocked sporting goods store here and walk out with 458 Winchester ammo and at a reasonable price.
Try that with the 470 Nitro Express or one of the more exotic big bore British cartridges. I expect you understand the price differential and availability issues.
Are you now maintaining that the British makers didn't develop their own cartridges to promote the gun makers ammo sales?
Jim