The longer it drags on the more chance China provides weapons to Iran assuming they aren’t somehow being snuck in
If nothing else it shows China how they match up against US weapons in battle and how to improve them.
Iran being the underdog despite its appalling treatment of its people could argue its just defending itself.
Plus what’s interesting is I think US allies (and their population) are actually experiencing schadenfreude in regards to the USA failing.
After Trumps treatment of everyone via tariffs, general insults they are somewhat fed up with the entitlement he expects