"Regime Change" has become a modern term for vassalization. We should not be surprised that countries with no reason to be a US vassal, and no long-term ties to the US refuse to remain vassals.

So then what would we achieve? nuclear material is cheap (10s of billions) relative to a multi-decade occupation (single digit trillions). It's undoubtedly true that Iran would revert to it's preferred form of government, geopolitical orientation, and nuclear capability once the US left.

How’d that plan work out in Iraq or Afghanistan, both much smaller, less armed countries? Decades and trillions spent, and what exactly did the US “win”?

Winning a war means achieving your political goals while preventing the enemy from achieving theirs. Most of the time, you've won the war when the enemy effectively admits they lost.

The lack of will to use sufficient force to win a war is fundamentally no different from not having that force in the first place. Both are equally real constraints on your ability to win the war.