Why should Iran agree to a new JCPOA agreement if there is a possibility that Trump will be elected?

I was just reading that the negotiations between Iran and the (original) signatories to the (original) JCPOA are close to finishing, and that there may be a new agreement soon.

Why would Iran agree? Last time they negotiated an agreement, Trump unilaterally ripped it up and took the US off of it. (and frankly, made things a whole lot worse).

Since Trump is (probably) going to run again in 2024, what’s the incentive for Iran to sign a new agreement? What’s to prevent Trump from shredding an agreement a second time?