If they end up getting attacked anyway, then their incentives change. If it looks like the West is going to try to overthrow their government anyway, they now have 100% incentive to just try to race to a nuke. They only need a delivery mechanism capable of putting it somewhere between Jerusalem and Tel Aviv to guarantee their safety.

So even though they've always wanted a nuke, they faced powerful incentives not to get close enough to it for the West and Israel to decide to just take out the regime. Attacking them shifts those incentives....

You're assuming rationality on their part. Thus far in their 46 or so years of existence they've...

*Created, funded, led and directed attacks by multiple terrorists around the world
*Taken over what amounts to 3 countries (Palestinian territories, Lebanon, Syria) with their terror proxies
*Provided safe harbor and trained terrorists in the making and deployment of Improvised Explosive Devices that were used in quantity on US Troops
*Arranged the bombing of the US Marine barracks in Lebanon

...and much, much more.

Trump's strategic calculus was this:

Do nothing:
-Maybe let the Israelis finish the job, knowing they don't have the capability the US military has
-But what if the Mullahs get desperate and decide to hang whatever they have on top of a ballistic missile? And aim it at Tel Aviv or Qatar?
-What if Israel can't quite push the mullahs to the brink and another Green revolution doesn't start? Missed opportunity.
-What if Israel inflicts what damage it can, but it's not enough and Iran waits them out? They still have their nuclear card to play.

Do something:
-Make an attack, try to take the imminent threat of a nuclear Iran off the table
-Sue for peace, but make it clear that the United States won't tolerate the status quo in the region

Trump went with the Do Something option.