Joe, Congress made it clear that the agreement wouldn’t be binding on a future president – didn’t the Republicans send a letter to Iran stating that to be the case? So everyone is fully aware that another president can back out, and a Republican president probably would.
As for your toothpaste comment, that’s not necessarily true. The US is involved in a huge amount of the business Iran does, whether directly or indirectly. Transactions between Iran and other countries can have problems if banks with branches in the US are involved – the US can freeze the funds. Some other countries will also go along with the US.