you're confusing things here.
Coil, Ex primals and dungeons have a high replayability. That's a given. Nothing to say on that.
Savage Coil however... Well, it doesn't bring more than regular Coil. You get nothing there once you have the title. Its replayability regarding the availability of regular Coil is non-existent. Same rewards, more difficulty (let's call it the path of least resistance).
People go there for the challenge, for the title eventually, but they have no reason to return there after (challenge beaten and title gotten) besides the fact that they might prefer it to regular coil, thus they're making the choice to redo it when there is nothing more they can get out of it.
That's the point. The Savage Coil was released with the "do it once and be done with it" mindset. Hildibrand... is kinda the same. Except for the fact that you can go there whenever you want with whoever you want and still beat the trials because its difficulty is way lower. However, once done, there is nothing more you can get out of it. It's done.
TL;DR : Though Savage Coil and Hildi's trials have nothing comparable in terms of difficulty, they have the same replayability : You have absolutely no reason from the game's view to go there again, you only chose to do so (for fun, for challenge, for whatever). Yet the game itself doesn't encourage you to replay it.
