The 450BM is a niche cartridge and appears to be dieing even in the AR. The 444 has been displaced recently by the revived 45/70, but is still more popular than the 450BM. The problem with all of these (45/70, 444, 450BM,...) is the lack of different rifles for them. The 45/70,444 are pretty much lever actions (yes there are some single shot 45/70s and even a pistol), the 450BM is pretty much AR only. Only the 350 is readily available in semi auto, bolt action and single shot. I would not be surprised is a lever action came out.
But the ballistics of the 450BM are superior to the 350 for large game and the 45/70 & 444 can take any game in the world. The 350 is basically a deer cartridge!