It hurts the language. People who could help improve and advocate for the language think twice before getting involved.
People want to feel good about their language of choice, see this use something else.
The conference used to offer different viewpoints, even if those viewpoints weren't the "official" ones by the Rust organization. It seems like they're changing this so only endorsed viewpoints will be represented in the conference, possibly leading to a stagnation of innovative ideas.
Time will tell what will happen, but when opposing viewpoints starts being hidden away, things tend to get relatively boring quickly.
On the other hand, as a conference attendee, I expect that the keynote talks reflect which direction Rust is going.
If they know that the person's point of view is totally different than the most likely outcome, having this speaker as keynote speaker is not a good choice.
(With all that aside, they should have thought about it before inviting this person, and not after)
It doesn't and shouldn't. The people in the Rust project will come and go, but the language will continue to be developed. Many contributers are not project members