Define failure. Failure is not necessarily a failure to achieve all objectives.
Keeping Iran from promoting terrorism thru Hamas, Hezbollah, or the Houthis, is a good thing.
But to your point about prolonged military action, if the US hasn't achieved regime change by the summer, its not a win but its not a loss. I expect to take Kharg island and control Iran's finances by the summer. What I don't know is if we can get the Jihadis and IRGC out of power by then. Victory can be defined many ways, regime change isn't one of them.
Trump's 4 objectives are:
Destroy Iran's missiles (in-progress)
Sink Iran's Navy (achieved)
Prevent Iran from developing nuclear weapons (finding 440 kg of uranium won't be easy)
Prevent Iran from funding terrorists (taking Kharg island and controlling Iran's finances would accomplish this)
A complete victory would be regime change without US boots on the ground hunting down the IRGC like we hunted down Saddam's criminal crew using a deck of cards.