Writing on Truth Social, Trump added: "I don't want that to happen, but it probably will. However, now that we have Complete and Total Regime Change, where different, smarter, and less radicalized minds prevail, maybe something revolutionarily wonderful can happen, WHO KNOWS?"
The ultimatum – set for 20:00 Washington DC time on Tuesday – demands Iran open the Strait of Hormuz and reach a deal over its nuclear programme, or face US strikes on its power stations, bridges and water desalination plants.
Trump had previously warned the country would not recover for "100 yeas".
It is the third time Trump has extended the deadline, having earlier promised he would not do so.
He said he delayed Monday's cutoff because he did not want to attack Iran "the day after Easter".
In Iran, authorities called on young people to form human chains around power stations.
President Masoud Pezeshkian said 14 million Iranians had responded to calls to volunteer to defend the country.
Meanwhile, the European Union condemned the threats.
"Any targeting of civilian infrastructure, namely energy facilities, is illegal and unacceptable," said European Council President António Costa.
The European Commission urged "maximum restraint" from all sides and called for full compliance with international humanitarian law.
The Strait of Hormuz carries around one fifth of global oil supplies in peacetime.
Tanker traffic through the waterway has been disrupted since US-Israeli strikes on Iran began on 28 February.
(ał)
Source: PAP