It's about time to add Coil story mode, isn't it ?

[saeedaisspecial]

and likely took about as much resource as a story mode coil would take. SE developed a separate version of an existing raid for a specific population of players. That's what the OP is asking for. When they did it for raiders, it was fine and dandy. When it's suggested for non-raiders, suddenly the sky is falling.

This is absolutely not the case, and precisely why your argument is flawed.
Savage Coil was the original difficulty of Coil. SE have stated at multiple points that they develop content harder than they release it, Savage Coil was them giving us a portion of that content released as initially designed. They've also said the original Crystal Tower was far, far harder too. No development time was actually spent in releasing Savage Coil at all, because it was already there.