A few things about this:
- The cupboard is not totally bare. We have enormous quantities of SDBs (Small Diameter Bombs) and JDAMs, both of which are aircraft-delivered precision weapons. The critical shortage is in standoff weapons (like JASSMs) that we'd use against air defense sites. Similarly we have lots of GMLRs rockets that are shorter-range but launched by the same land-based platforms that can launch PrSMs.
- The numbers on air defense missiles (Patriot, THAAD, SM-3, and SM-6) are really bad. Like for any realistic war against a near-peer nation like China we'd probably need five times as many missiles stockpiled as the (optimistic) targets in the above table show. We'd need even more if we were simultaneously fighting a war in the Middle East and trying to supply friends and allies as well. If we decided that the homeland needed defending too we'd need even more.
- THAAD and SM-6 are the most effective anti-ballistic missiles we have. They are also far and away the most expensive (e.g. about $10 million to $12 million each).
Supposedly there is an updated variant of the Patriot that uses modern microelectronics and is much less expensive (about $2 million per missile as opposed to $5 million for the current top-of-the-line version).
Missiles with solid rocket motors (all of the ones described above, as well as things like Javelins and Stinger missiles) have a finite shelf life before the rocket fuel degrades and no longer works (and sometimes just explode). Most of the Patriots we shipped to Ukraine were ones at the end of their service life so knowing the Ukrainians would likely be shooting those missiles saved us the money on safely disposing of them.
We don't really know how many ballistic weapons Iran has left, but it is safe to bet that it is far more than we have air defense missiles in-theater. The missiles that can hit targets in the Persian Gulf region are
far more abundant than longer-range ones that can hit Diego Garcia. It is also safe to assume that the number of Shaheed drones they have and can make are far larger than what we can currently defend against.
Until we have effective anti-drone systems, an effective doctrine for using those systems, and people trained on the systems and the doctrine any of our people in the Gulf are just being used as targets. At this point the Ukrainians are closest having the whole package, and even they struggle in a lot of situations. So we have quite a bit of work to do.
I'd emphasize again that doctrine and training are just as important as hardware. You can have the world's greatest hardware and if you have crap doctrine and nobody trained on the weapons or the crap doctrine you will accomplish nothing but getting good kids killed for no reason.