I think if we had wanted to topple the Iranian regime we could have awhile ago, we just lacked a president with the will to do so, and Iran was useful in fighting Isis.
However Iraq, Egypt, Libya showed that sometimes it's better to have a functioning but unfriendly government in place, than a chaotic vacuum for extremists to fill.
Now that Isis is pretty much decimated the current Iranian government is less of a neccesity.
