I'm guessing he is afraid that she may come across as an angry black woman. Granted I think she could effectively shut down Trump in debates and make him look like the fool he is.
I am of the opinion that right now and for the foreseeable future there is no woman who can shut down Trump in a debate and
not lose male voters. It's too close to playing the role of the nagging shrew, and since a lot of Trump voters associate themselves with him (cult of personality and all that), it's a losing proposition. I think his "nasty woman" attack is how a lot of white males feel about the women in their lives, and he'd just roll it out again.
Then again, I'm also of the opninion that Trump won't be the Republican candidate in 2020, so it's kind of irrelevant.
I think maybe you are for some reason not recognizing the non-legislative consequences of a Nazi being president.
(Hint: a woman was killed in a terrorist attack)
*nods* Regardless of the policy upsides, it will wreck US politics if Trump isn't punished for his views, speech and actions. It would fully normalise a lot of things, and it wouldn't just be political or legislative norms that were changed for the worse.