The problem is their leadership is more likely to die in a forever war against the US.
Sure, 2003 Iraq may have been a huge mistake for the US - but Saddam isn't around to celebrate it and it's not like the Iraqi Republican Guard is exactly doing any victory parades over their crushing defeat of US armored thunder-runs or aerial grid-square annihilation.
This also doesn't solve the internal issues, because "bros let's all die fighting against the US and Israel" is just as likely to cause internal instability as the alternative.
Sure, Iran, especially the IRGC, is full of fundies who would gladly die in a holy war against infidels and gladly live in fucking caves and small villages in the hills for the next two decades - but is every military commander left standing and is every civilian leader feeling like doing that?