I do think Irelia is a very strong top laner, but definitely not the best top laner. She does deserve a bit nerf, but 40secs on her Ult at rank 1 is overdoing it. Her Ult provides good damage and sustain. Especially at early stage of the game, not to mention it's her ONLY skill does damage to multiple targets.
Moreover, I believe Triforce buff also gave a lot of other champions similar advantage. Such as Jax, and Jax is not taking any nerf from the patch. Jax has been a very strong champion for a long time due to his skill kit. Darius 1-hit turned Max Stacks Armor Pen is not taking any nerf at all. Garen...oh, he's just terrible lane bully in the right hand. All these champions aren't taking any or much nerf in recent patches.
Honestly, I think this is like Poppy nerf. When someone is rather popular and strong. Riot wants to nerf them so other champions could get more chances to be picked.
Popularity is a biggest reason behind the nerf. Overpowering other champions is just a small part. Riot just doesn't allow popular yet strong champions to exist. Look at Reworked Poppy's win rate drop and chance to be played top lane after the nerf patch. Irelia will suffer the same fate, not because of her overpowering status. It's because of her looks and popularity. Riot is telling us, you can't have it all in real life; therefore, you can't have it all in the game as well.