The problem with DEI isn't diversity. Diversity is beneficial, for the reasons you state.

The problem with DEI isn't with equity (construed as "fairness and same opportunities"). Equity (defined thusly) is beneficial, for the reasons you state.

The problem with DEI isn't with inclusion. Inclusion is beneficial, for the reasons you state.

The problem with DEI is the way in which people have (sometimes) tried to reach diversity, equity, and inclusion.

Do you approach diversity goals by having better outreach, or by failing to seriously consider candidates of the group you consider over-represented?

Do you try to reach equity (as fairness and the same opportunities) by talking about implicit bias and the psychological benefits to whites of racism and leave it to people to figure out on their own what to do with those claims, or do you provide concrete strategies for how to structure evaluations fairly?

Do you increase inclusiveness by saying one group--white people, let's say--cannot understand the experience of another group--black people, perhaps--and furthermore that if white people were to speak on a topic that concerns black people, that would be "centering white voices" and they should therefore self-censor, or by pointing out how we are all people with our differences and perspectives and should invite everyone to speak?

The goal of DEI resonates with most people outside of hardcore white nationalists; the need may be inadequately understood by some; but others may understand the need yet find the methods lacking.

People who support DEI all too often respond to the last kind of critique as if it's being made by the first group. And it's true, actually, that the first group will gladly try to make it sound like they object to methods, if they think those claims will find any purchase. But it's not true that methods are unimportant.

Excluding a diversity of ideas about how to accomplish DEI would be ironic in the extreme. But, alas, it happens.