We make them earn it. Here's a typical email I send in response to such requests:
"We do sometimes recycle usernames when they're well and truly dormant, i.e. no logins for many years. But we only do this for established community members.
What I usually tell people in situations like this is to create a new account and build up a track record of being a good community contributor, and that if they do that, they're welcome to email again in the future and ask us to recycle the old username for them. We can always rename your account at that point."
If someone is already a good community contributor and has already built up such a track record, we're more likely to give them what they want. But only if the username they're asking for has shown no sign of life for many years.
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Edit: the above comment generated quite a few emails, which has forced me to think about what makes me uncomfortable about this. Here it is:
Obvious/hot usernames don't really fit the intended spirit of this site, which is (intellectual) curiosity: https://hn.algolia.com/?dateRange=all&page=0&prefix=true&sor.... That spirit is more likely to come up with a new, creative username than to seek the one that's better for branding or vanity.
Branding is about being obvious and repeating things. That's out of sync with the spirit of this site. Similarly, vanity—though perfectly human and something we all have—is not particularly sympatico with curiosity. Put those together and you get the worst of both: "personal branding", which is particularly boring and uncurious.
Curious conversation is about some $thing that is interesting. It is not about $me, and when $thing is merely a means to $me (or $my-startup or $buy-what-im-selling), that's lame. You can usually feel it and smell it in the content, too, because the content wasn't produced out of interest or for its own sake.
Optimizing HN for curiosity (see link above) is a constant fight against the forces of promotion, which forever seek to pull people's attention away from things that are actually interesting and glue it instead to ulterior matters like marketing and messaging and sales. I'm not saying those things are bad in any absolute sense, but they're bad relative to the mandate of this site.
So this whole thing of "can I please have me this generic / obvious / presumably-high-status username that hasn't posted anything since 2008" makes me feel like someone doesn't really get what HN is for, and I get queasy.