The iPhone and any app on it can be exploited. The microphone can be remotely turned on, it's location can be tracked, etc. etc. We can evaluate the likelihood

The iPhone and any app on it can be exploited. The microphone can be remotely turned on, it's location can be tracked, etc. etc. We can evaluate the likelihood of attack based on the attacker's return on investment: If there's a $1 million secret and the attacker can get it for $0.1 million, that's a good deal; if it costs the attacker $2 million, then why bother? The value of accessing POTUS' information can't be overstated. It's the literal survival of nations, trillions upon trillions of dollars, the very course of the history of civilization. If foreign powers can access it, even if it costs them billions of dollars, they will. Even allies do it - the U.S. was caught hacking the German Chancellor's phone in the last several years. The same goes for others with the resources and the willingness to break the law, such as organized crime and less scrupulous buinsesspeople - you could easily make billions on the markets by knowing Trump's plans. And if your competitor knows and you don't, you are at a disadvantage. Also, it's a good source for blackmailing POTUS (an incredible but now conceivable thought) and for gaining similar information about the powerful people he meets with.