While conflict of interest rules are in part designed to prevent personal enrichment, they have far broader purposes.
They are as much about protecting the quality and legitimacy of decisions.
Imagine those scenarios:
President Trump orders US troops into a country with a large Trump economic interest, and some refuse to go on the grounds that they do not want to risk their lives in order to project his hotels.
Trump follows up on his apparent change on climate policy, and decides to be aggressive in reducing carbon emissions, and some refuse to comply on the grounds that his seafront properties are increasing in value because of the change.
Trump orders torture to find out more on a terrorism threat in a city with a major Trump investment, and the operatives involved refuse. What can be done to those operatives? Would later litigation by the victims go further than the current Guantanamo round, when economic interest of the President could be part of the cause of action?
Trump pushes an open-skies agreement with a country that has Trump investments, and the airlines which are going to lose monopoly status challenge the process by which the decision is made.
To reiterate, the problem is legitimacy and quality. For an incoming President already burdened by a well earned reputation for ignorance and lack of attention to detail, to have questions raised about the legitimacy of every decision in which he is involved — and to increase the chance of litigation involving questions about this role in every such decision, would be a nightmare.
Moreover, he won in part because people believed that he was not motivated by self-interest. Any proof otherwise — particularly one that seemed to go against his pledges and the interests of his base, has the potential to be fatal.
So, if he has any sense, he puts in place a system in which the chance of such conflict is zero. Meaning, in fact that he has to sell his current interests, and not in a way that he can get them back when he resigns, is impeached, or loses reelection.