You have it exactly backward. The law is clear that "campaign expenses" cannot have any personal component. That makes sense, because the focus of the law is to prevent candidates from calling things "campaign expenses" that are actually for personal benefit. John Edwards was prosecuted for using campaign funds to keep his mistress silent because hiding an affair has a personal component in addition to a political component.
If Trump had been charged with a campaign-finance violation, his straightforward defense under well-established precedent would be that paying off Stormy Daniels had a personal component (avoiding personal embarrassment and his wife finding out), in addition to whatever effect it had on the campaign. The prosecution would have been required to prove beyond a reasonable doubt that Trump's marriage was so dysfunctional that the only reason he would have paid off Stormy Daniels was the election.
That's why prosecutors didn't charge him with a campaign finance violation.
You also can't have your personal lawyer take out a loan and spend it on expenses for your campaign and not report that as a campaign expense.
If you're running for public office, you can't commit fraud to hide things from the electorate because you don't want them to find out. That can't be allowed in free and fair elections. If you get elected doing that, people have every reason to question your legitimacy. If you get caught and lie about it, expect people not to trust you. If you get charged with a crime related to that conduct, don't be surprised when a jury finds you guilty, because it's shady af. This is all very straightforward stuff.
