Yeah, I agree, but they're two different archtypes. Jiraiya is definitely better for the open battlefield. With synchronization from Gambabuta, he can set entire armies of fire. With Sage Mode, he can sink entire armies with Yomi Numa. And yet if this manga has taught us one thing, it's that being able to kill loads isn't the most valuable ability. It's being able to take down key players. For most countries, once the Kages & Jinchūriki are down, they're done. Jōnin can't even measure up.
And while Itachi is weaker in mass destruction, he excels enormously in stealth tactics. Remember how Shikaku said only Itachi could control many ninja with genjutsu? How Kabuto said Itachi could even control him with genjutsu? That ability to control powerful ninja is huge, and even more importantly, Itachi is built like an assassin. Where Jiraiya has massive techniques that shoot like a bazooka, Itachi has a bunch of laser-like techniques. He can't spam them, but they can bring down higher quality opponents, and at the end of the day, that's really what matters in ninja war imo.