You make a good point that it benefits a lot of people to know where the high profile members went, but maybe there's a better way to do it.

If there is some unique perspective on HN that can apply to everybody then, sure, go ahead and post it to HN as a blog post. However, any generic I'm-leaving-HN messages could maybe go into a category of "Leaving HN: ..." posts (instead of a link). These could still hit the front page, but there would be a clear distinction between this community news and other news.