Couple of things; don't overlook that the 9,3x62 is a standard-length cartridge, meaning a lighter carry rifle compared to .338 Winchester and .375H&H. It's considered a relatively mild cartridge to shoot which also helps put meat on the table. Not having to use an exclusive magnum-length Mauser action would have saved quite a bit of money in the 1930's and allowed more selection of makes. This alone could account for its' popularity.

Also I'm not sure I agree that rimmed cartridges have lost their need in the 21st century. When they're used in a break-open gun such as a Drilling, the extraction is more or less assured. However rimless cartridges require various contrivances like a small spring-loaded pin to engage in the extractor slot. I'm sure these work well enough most of the time, but for peace of mind, a simple rim has got to be the better option. In some cases, rifles will come set up for both options, especially if there might be issues with getting ammo. So for instance, I was offered a 5,6x57 rifle/shotgun (a BBF), that would accept rimmed or rimless ammo without needing a change of extractor.

I think a Drilling in 7x65R would have a ready market, but one in 7x64 rimless might meet some resistance.