How does this guarantee a political and strategic win for Iran?

Iran was already teetering on the edge of being a failed state: socially, economically, environmentally, and agriculturally. Iran is expending expensive ballistic missiles to force those THAAD and Arrow shoot-downs. Yes, they're winning the shot exchange ratio, but their economy is orders of magnitude smaller than the US. Besides, unlike the Gulf states, the US and Israel are not just sitting around playing defense. They are systematically destroying substantial fractions of the Iranian war machine and have both threatened and attacked domestic and international energy production, the lifeblood of the Iranian economy.

The only true winner of this war, however it shakes out in the end, is Russia. All of the Middle Eastern powers aligned with the US are going to be desperate to rebuild their interceptor stockpiles and will surely get priority over Ukraine, likely for a very long time as the production rates are very low as you've pointed out. Plus, Russian gas and oil are worth a lot more than they were prior to this war, and are being allowed to trade more openly as well.