Well US and its allies may run out of interceptors even before Iran runs out of BMs. Even if Iran does run out of BMs, they can endlessly produce the killer drones. Those are not very complex or expensive but they can keep US forces out of their bases indefinitely, disrupt entire oil supply, reduce US interests and personal presence in the region and can end the war on Iran's terms.
The cost of war, ammunition, allies pressure, US investments, interests in region, US consulates, embassies, CIA stations in whole of the region is on stand still. Even US companies like Amazon were targeted. US do not have time with itself. Its warships which have exhausted their missiles can't even reload at sea. Iran has time with itself. All that noise of regime change, cutting head of the snake, communication jamming, EW and what not as per US / Israeli statements seems to have not much effect over Iran. US was hoping a quick action by first massive strikes to subdue Iran but they have no solution now. What they really wanted to achieve? Make Iran less of a threat by regime change ? or by finishing its offensive capabilities? Well, Iran's regime is going nowhere and it has become and will become even bigger of a threat then it ever was. Whenver its over, they will prepare for next confrontation with even more determination and furiosity.
Iran has built itself with this kind of war. It has enormous stamina to absorb bombing campaigns. It's command structure is decentralized that's why they are able to hit back so hard at such wide scale without even blinking. US spectacularly failed to eliminate Taliban in 20 years and Trump thinks it can remove IRGC ? Its not South America.