I think it would have also been perfectly professional to decline in that case, since it's clearly poor planning on the part of the folks organizing the talk. And demoting the keynote speaker is even worse - especially when in this instance something like scheduling doesn't seem to be the excuse

It is always professional to remove yourself from unprofessional situations.

Was it a talk that you submitted on your own choice, or were you asked to give the original talk? In this case, the speaker was asked to keynote-speak, when they had not planned on doing so.

How long was the talk you originally wrote, perhaps they thought there wasn't enough content for a full one (playing advocate)?

If there are malicious actors, by "acting professionally" (probably meaning to ignore your emotions) you are allowing those who are malicious, and do not act professionally, to continue doing that.

Others acting professionally is the best outcome for those who do not do so.

Melodrama from the blog post aside, publicly downgrading a talk for an industry wide conference is very much a slap to a speaker's face.

"Hey we think you have great ideas to share, and would really like you to share them."

On short notice: "One of our team members thinks your ideas aren't that important after all, we are going to remove you from the headline. Please keep it short kthxbye."

It shows bad planning, bad organization, lack of cohesion, and outright disrespect of your professional time. You may choose to accept that slap to your face (signaling that it's okay to do that to you), but there's nothing unprofessional about rejecting the attendance and rightfully criticizing the organizers for their rudeness.

Make a joke about it in the presentation. This talk was downgraded to get less visibility. So I’ve renamed the talk to “What Rust leadership doesn’t want you to know”.

I agree that it’s bad planning. And in particular, the reasons of “discomfort” are really terrible. However, it’s a very small hill to die on. Like, a mound.

I hope you cried, left your field forever and wrote 5 blogpost where you whine about it and reposted on every possibile social platform because that’s seems to be the right way to handle it