You make it seem as though AJAX is some unstable, bleeding edge experimental feature that no one ever uses. There's no danger of sacrificing backwards compatibility, or of browsers no longer supporting AJAX in the future.

However, Hacker News does use AJAX, and I don't think it even uses whatever img tag trick OP is referring to. The only img tags on the page are the icon and the vote arrows. If it's there, I couldn't find it.

But read the javascript yourself - AJAX support is just a single, simple function. No unit tests and QA team, no depending on some flaky maintainer. Maybe they do unit tests, I don't know, this place is kind of a black box sometimes.