All of them are great cartridges. Here is my opinion.

375 H&H Flanged
Great round that lost out to the 9.3 x 74R due to ammo
and brass unavailability as well as the fact that the
Euros liked the 9.3 and of course this was always available
and does the same job. So in the end, the market trumped nostalgia in numbers but their will always be a following
for the 375 H&H Flanged.

400 H&H. Too proprietary to take off.

450 Rigby. Too many other alternatives already out there
to take off and unlikely tobe taken up by the major manufaturers, especially now the 404, 416 and 500J and 505 Gibbs are available in Factory CZ's and other guns.

500/416 NE
I was always surprised this didn't take off.