I think one of the attributes of the 400 is the lesser amount of recoil than the 450's. It has always been 'good enough' and combined with the lighter weight, good balance and lighter recoil has worked well for 100 years.

I think that when you consider rifle weight, recoil and the better velocity you find that the 500/416 is more a substitute for a 450/470. Flatter shooting, if that is important, and as hard a hitter with the increased velocity.