It absolutely can conquer Iran lol it's a much stronger much better funded army - the issue is _holding_ Iran and what the point actually is. Even in conflicts like Vietnam when the other side had a superpower backing them (or Afghanistan when the Soviets attacked it) the issue isn't grabbing the territory it's the fact holding territory doesn't actually accomplish much and the strategic goal (not having a nuclear theocracy there) isn't met long term.
Those people also hate the IRGC; the recent popular uprising was only suppressed by murdering ten thousand people. The regime is not as stable as you think.
Just war [0] proponents will argue both. For instance, targeting civilians/civilian infrastructure is gravely immoral (the decimation part), but also any armed conflict is unlikely to be successful and thus would be immoral on the grounds that there is no reasonable chance of success (the conquer part). But frankly, in this case, we don't even meet most jus ad bellum criteria, let along jus in bello.
Not really raised to hate the US, but they do learn in school that the sarin Saddam Hussein used on their parents and grandparents were from US stocks. Which is factually true, even if used as propaganda.
Decimate? Sure. Conquer? Absolutely not.
Iran is a huge country with dispersed command and control and generations of people raised to hate the U.S.