As I asked, if Iran is as totally destroyed as Trump asserts, what is the purpose of Trump's ultimatum?

Time is running out for Trump, not for Iran.  The last time an American president took America to war Constitutionally was 1941 when Congress gave the executive branch permission to enter the war with the Constitutionally required Congressional Declaration of War. As time went by Congress finally responded to presidential decisions to go to war without a Congressional declaration of war not by impeaching the President, which should have been done in order to protect the Constitutional political order and separation of powers, but by requiring the president who initiates military action without Congressional approval to come to Congress with a deadline of 60 days after initiating military action for congressional approval to continue the military action. 

In other words, Congress failed to defend the Constitution's Separation of Powers by allowing the executive branch to exercise a power it does not have to go to war and, afterward, to come to Congress for approval. In the past Congress has rubber-stamped the President's decision.

But this time it is different. Polls indicate that a majority of Americans do not share Trump's concern  about the Iranian threat to America.  They do not support Netanyahu's war.  Even many American Jews do not support the war.

On April 2 the Times of Israel reported that "the US Democratic National Committee is set to consider a resolution at a meeting next week that "condemns the growing influence" of AIPAC.  This is extraordinary considering that in the US Senate there are 9 Jewish Democrats and no Republican ones and that of the 25 Jews in the House of Representatives, 21 are Democrats.  https://www.timesofisrael.com/democrats-to-weigh-resolution-against-aipac-fueling-concerns-about-undercurrent-of-antisemitism/ 

The Times of Israel reports that: "A recent NBC poll found that 57% of Democratic voters have a negative view of Israel, compared to 13% who have a positive view of the country. Meanwhile, a growing number of the party's congressional candidates—and politicians thought to be seeking its 2028 presidential nomination—are swearing off AIPAC, and crossing its red line of supporting conditions on military aid to Israel."

What Trump has done is to ally the American Democrat Party against Israel and the Republicans with Israel  Or to put it more correctly with the current Zionist government of Netanyahu.