The percentage of people who would pay for it, probably wouldn't make up for the dev time to implement it.
Once upon a time the HN userbase and the reddit user base weren't a million miles apart, those days are long gone, reddit is mainstream and mainstream consumers don't care that much about ads, the new UI or everything else this thread is up in arms about, it's still the defacto internet forum and nothing I've seen looks likely to replace it.
