AD by themselves only buys you time, Israel has a blank check to the best western AD and they're still getting bombed left right center, Ukraine has been given countless Patriots and they're getting bombed just the same.To be honest, even if Iran has S-400, HQ-9 or even J-10, what difference does it make when Mossad could destroy them on the ground with Spike ATGM? The fact that Russia readily provide S-400 systems to India, yet is hesitant to deliver even S-300 to Iran tell volumes. Recall that in 2024, the 3 S-300 Russia did provide to Iran were destroyed by Israel after internal software attack rendered them useless.
Also, hasn't Iran shown a multitude of AD systems on parades? What happened to their Raad, Bavar 373, Arman, Sayyad, Zubin air defense systems? It's evident that while Russia/China were hesitant with providing Iran with AD systems, they were quite open with providing Iran with underlying technological know-how.
Unless Iran can afford a full suite of Chinese aerial assets, and I don't just mean what PAF has but much, much larger in quantity and scope, enough to fight for air dominance against what US gives to Israel and by extention USAF itself, investing more IADS will at most buy you an extra month before you run out, during and after which they'd have no ability to counterstrike.
Iran isn't symmetrical to Israel and AIPAC controlled US and shouldn't fight symmetrically. They should have invested much more in internal security and should have spent more effort securing alliances with China and Russia rather than trying to make peace with the west who wants them dead for religious reasons. But more IADS would have been a bad investment either way.