A few things:

• There's a Venn diagram of "getting older" and "forgetting what year it is, referencing people that have been dead for decades, nodding off mid meeting, 2 a.m. tweet tirades, mood swings, TIA symptoms, recurring MRIs & cognitive tests, etc..." and the overlap is pretty slim
• Having access to world class healthcare doesn't guarantee anything when the patient is a powerful narcissist. Steve Jobs died from a very curable cancer because he thought he was too smart to die. If Trump's doctors tell him to cut out cheese burgers and diet cokes do you think he'll listen?
• Unlike a simple [death/remission] cancer prognosis, dying isn't the only ending for the potential systemic issues he could be suffering from. He could deteriorate in any number of ways that can't be smoothed over with PR and leave him unable to hold office. Think paralyzed, nonverbal, coma, etc...
• The man clearly doesn't have the drive and ambition to care about his longevity. His goal is to siphon money while continuing to live an opulent lifestyle and die in office (however long that may be). Fear of death isn't the same thing as will to live, especially as the decades add up.

Just look at the guy, his healthiest attributes are golfing and not smoking/drinking. Assuming he's just obese and in otherwise perfect health and golfs 40 hours a week (without a cart), a calculator still puts his life expectancy in the mid 80s. Are you really betting that there's some medical dark arts that will keep him functioning indefinitely? Plenty of richer and more powerful men have died in the same situation with comparable healthcare.