Afghanistan, Iraq, and Syria were costly blunders. They cost America lots of men, money, and prestige. A war with Iran could cost the US more of all of these things than the other elective regime change wars did. Iran has many more people and is much larger than the other theaters are:
The US won’t be fighting against a Sunni or Alawite minority, either. It will be fighting against a Shia majority while presumably continuing to simultaneously occupy another Shia-majority, Iranian-friendly Iraq.
Donald Trump was not elected to extend forever wars, he was elected to end them.