I think the 300 H&H dropping away is different for a couple of reasons.

It was the "velocity era", the 300 H&Hs were being rechambered to 300 Wby and 300 AckImp etc.

As best I can remember factory claimed ballistics for the 300 H&H were something like 2850 for the 180s and 3100 plus for the 150s. The 300 Win came out with initial factory claims of 3070 and I think 3390 or 3400 for 150 grainers.

In addition it is about 43 years since the 300 Win was introduced and that also means the 375 H&H has been around for a additional 43 years.

In the case of the 375 H&H, people just don't seem t interested in faster versions. I would bet plenty that Wby in those early days rechambered very few 375 H&Hs to 375 Wby. In fact Wby dumped the 375 Wby and went to the 378 Wby. Today, Wby does not even chamber the 375 Wby in a standard issue Wby rifle but Wby do chamber the 375 H&H in their Synthetic models.

Personally, I think the 375 H&H is seen as the "real thing"

On the other hand, the good news for Ruger and Hornady is that my track record on making such predictions is very close to being 100% wrong

Mike