The Iran War never looked good on paper. The only people who thought it would succeed were Trump and the cast of characters he surrounded himself with. I doubt if many congressional Republican chickenhawks thought it would succeed.

The only way to oust the regime is with ground troops, ripping out the Revolutionary Guard and its tentacles. For all its corruption, Iran is far from a failed state, and there aren't factions waiting in the wings, ready and willing to take over the government with force. (There are political factions, to be sure, but they're already integrated into the government, though without leverage over the Revolutionary Guard.) The only armed group remotely capable of even trying would be the Kurds, but the US and in particular Trump screwed them over in the past, multiple times. Even if they thought they could go it alone (which they couldn't), there was zero chance they were going to enter the fray without the US committing itself fully with their own invasion force (i.e. success was guaranteed), because failure would mean ethnic Kurds would be extirpated from Iran, and might induce Iraq and Syria to revisit the question of Kurdish loyalty to their own states. And, indeed, Kurdish groups took a wait and see approach, assembling some forces but waiting to see how the US played their cards.