The good news is you only need to clear it once and the reward is Coil2.

I do agree that when coil3 comes out they should add a story mode t5 so people can progress to Coil2 via an easier battle.

The problem with t5 and t9 for that matter are the one shot mechanics. Even at i250 those battles will wipe groups that aren't versed in the fight due to cheap mechanics. As the game ages there will be less and less people doing those fights as well.