UKR = entire country of +40m is on the battlefront so they can do total war mobilized homefront distributed system... so can Iran. But it's very different for force projecting security guarantor US - can't convince paying protectorates to pivot total war defense posture in peacetime, that's what they bribe US not to do.

And ultimately whatever model of distributed lethality / survivability (which US planning foresaw) is less relevant that US global commitments requires high end hardware that has to be rotated / propositioned selectively, and sustainable only in limited numbers vs adversaries mobilized on total war.

But the fundamental problem is US adversaries are catching up on precision strike complex. Iran isn't asymmetric warfare, but restoration of symmetry. It's not so much US getting weaker as adversaries getting stronger, and without monopoly over mass precision strike (which naval / air superiority / supremacy is only delivery platform), US expeditionary mode simply on the losing side of many local attrition scenarios. Ultimately all US adversaries will gain commoditized local precision strike (even deadlier if bundled with high end ISR), at varying scales due to proliferation requiring persistence across global theatres US simply doesn't have numbers/logistics for.

TLDR: US expeditionary model is bunch of goons with rifles in trucks, driving around neighbourhood where everyone had knives that could not get in range. The second everyone else buys guns, then rifles, the expeditionary model breaks.