He fought Hashirama and gained this power. Madara planned this from the start. Since he actually planned this out and actually worked for it by fighting Hashirama (which is no easy task), Madara fairly earned that power. Plus, that power was needed for him to reach his full dojutsu potential (rinnegan). Finally, Madara spams susanoo all the time. Therefore he does rely on his own original power. He just uses other powers that he has earned in conjunction. Also, suicide and sacrificial moves don't count as actual battle skills. They are just last ditch efforts. If you have to commit suicide or sacrifice half of your power (which is one sharingan eye in this case) just to beat an opponent, you know that your opponent is stronger than you. A real win is a win that doesn't involve sacrifice. It is for this reason that Izanami can not be considered as a skill that puts Itachi over anyone. Plus, Izanami is an Uchiha jutsu (not an Itachi jutsu). Therefore it is not a testament of Itachi's skill.