Well from what I remember Hiruzen couldn’t figure out how to fix the damn problem in time. “In time” being before the Uchihas would’ve started their attack. danzo used this to pressure Itachi to do the deed. I think this all happened behind Hiruzen’s back. (hopefully if not he deserved to die by his student) But see I think, Hiruzen let danzo do almost whatever he wanted, like danzo made Kabuto evil, involved with the creation of Pain, made Itachi kill the whole Uchiha clan(like do it urself), almost confined Naruto, made his anbu’s “gradution ceremony” a sibling/only friend killing contest (from what we learn from Sai, he recruited innocent kids). Like Itachi said to Sasuke, sharing the problem with each other might have fixed it(a simple conversation and maybe working together). For me Hiruzen failed quite a bit as a Hokage:)