I have shot a 375 H&H, my friend had a 700 Remington in that chambering. We shot it off the bench, and prone, with some handloads and factory rounds he had.
I was surprised that it was not at all hard on the shoulder, even though the barrel was pretty thin. And in spite of that thin barrel, the totally stock rifle shot well. I think the 375 H&H is pretty easy to please with the load recipes. We even shot the rifle at 300 yards (prone, off bags) and it literally shot MOA and under at that range with a couple different bullet weights.
If I wanted a big bore hunting rifle, I'd definitely consider a 375 H&H, but I'd probably want it in a Ruger number 1 or something nice like that.
But I think that the 30-06 just covers more "niches" than any other cartridge out there. If the .308 win had come first, it might be number one in worldwide sales.
What we need to realize is that there are valid, sensible reasons why so many folks continue to buy the 30-06. Put simply, it works. If it didn't work as well as it does, it would have fallen out of favor decades ago...
Most of us have favorite cartridges that we can list a litany of reasons for why we like them. But we're rifle aficionados. For folks who just want something that works, they'll choose the 30-06 over and over again.
It's the same thing with the .357 Magnum. Is it the most powerful handgun cartridge? No. Will it kill an elephan t? I wouldn't want to try it. But the .357 is well known, and trusted, and the phrase "three fifty seven magnum" just rolls off the tongue of the average schmo. That's another cartridge that will probably never die.
If power is what we're considering as the qualifying factor as "king," the H&H is of course out as well.
I think the "king" should be elected as the cartridge that meets the needs of most folks looking for a rifle. The 30-06 has been doing that for more than 100 years, and will likely continue to do so.
It's kind of like Richard Petty. Many will bash him, and make excuses for all of his nascar victories... and carry on and on about why Earnardt was a better driver... but the truth is Petty has won over 200 races, and will continue to be the "King." Like it lump it.
Dan