That president would be hated by big business, they could and would sink them. Also, half of the US is so far down the propaganda rabbithole that they either wouldn't believe that it's fixed or they would be convinced that it's bad that it was fixed (e.g. anti-vaccine people).
And finally, a US president is only legally allowed to have 2 terms (though Trump is definitely going for "dictator for life"), and it's not really possible to completely overhaul a healthcare system in a way that people will notice positively in their daily lives in just 4 years; so by the time when even the people who were always voting for the other side would notice that healthcare is good now, there's already a different person on the ballot.