At least some types of UI changes would affect content quality. For example changing the size and color of the up and down arrows could have a big impact on voting frequency. Making the submit link more or less prominent could have a big impact on submission frequency, as well as providing tools like submission bookmarklets. And if at some point the majority of HN usage is mobile, it'll really change the sort of stories that people upvote.
It's not clear which direction you would want those metrics to move to improve quality, but it at least seems like UI vs quality is not a zero-sum tradeoff.
But would increasing voting frequency make the content better? It seems to me equally likely that it would make it worse. Ditto for having a more prominent submit link.
There is a submission bookmarklet. There's a link to it at the bottom of the page.
Yes, it seems equally likely that it would make things worse. But it seems unlikely that it would have no effect. It is unlikely that these are the optimal shapes of the up and down buttons.
For what it's worth, I think the Google way to solve this would be to figure out which metrics you can associate with quality and A/B test the impact of the UI changes. Setting up an A/B framework might be more work than you want to put in. But there might be a UI that encourages higher quality that you could discover empirically.
It's not clear which direction you would want those metrics to move to improve quality, but it at least seems like UI vs quality is not a zero-sum tradeoff.