What you say about fat, low taper cases producing the same velocities as their tapered counterparts at lower pressures is correct. But that is not the only consideration in good case design, nor is it the most important. If it were, then every cartridge designed and released in the last 50 years would look like one of the Weatherby magnums, the STW, the 8mm Remington Magnum, or the Winchester Super Short magnums. Of course, that is not what has happened.

The 400 H&H is a very good design. Obviously, it wasn't designed to produce more velocity than every other .400 cartridge. Judging by what was said about it, it was designed to produce velocity within a certain range and to do that at a reasonable pressure while providing a safe margin for pressure increases encountered when hunting in high temperature environments. It was also designed on H&H's famous long magnum platform, making it a perfect candidate for rebarreling of .300 H&H and .375 H&H rifles. Further, the long taper is reminiscent of the 300 H&H, a very successful round that has a long and well deserved reputation for feeding "like butter".

Various cartridges exist for various reasons. The .400 H&H does not exist to answer the requirement for the fasted possible .400 cartridge. I could go on, but I won't. Suffice to say the points above were enough for me to have a .375 H&H rifle converted to .400 H&H.

The bottom line, the .400 H&H exists in its present design because that design had sufficient raison d'être for H&H to make it that way.