Normal mode can essentially be considered as filler content since it's shelf life isn't that long beyond the gated gearing process. If they had say, FCoB-tier normal mode and savage mode, the former would still last a bit longer, for both the process of gearing and content for players to tackle, while at the same time, savage still present a challenge without making people burnout if they're attempting it over the course of 4 months (when a month or two is spent on the previous tier).
To ride on what you're asying, savage mode being the only raid tier, and even if people do clear it, all that's let is the gearing process behind it then it becomes exhausted. Content needs to have shelf life, but at the same time it shouldn't be overly inacessible for the general population to tide them over until the next content patch.