There's a reason why hashirama got breakthroughs while itachi no.

Because hashirama is weaker and by a lot.

Hashirama initial idea was to be a ninja to make viable water and wind main as main move 1 damage dealers, but failed because his set was not good enough mostly because doesn't buff them and people preferred to use him as move 1, but since he isn't immune to debuffs or chases and his cd is too high he failed also on that side.

So they thought was just better to think about him as a good support ninja.

Itachi susano'o honestly speaking fails only on one side: mystery. All his other set is still good. An eventual trial for him that give him shisui mystery would make him go back to his past glory, but beware that he is still pretty solid.

The team itachi susano'o - madara founder/konan taisho - shisui - earth main is still perfectly viable.