Newest was replaced with oldest because it doesn't break bottom up viewing of questions that have multiple pages of answers. There was a bit of debate about which to have, since they're basically redundant. Others have expressed profound grief over its absence, but we're not very likely to change it.
As this would affect every site, this is something you'd probably want to propose on Meta SO as a feature request if you have a strong use case for changing it back. I'm declining it here to keep it from appearing as an open feature, that doesn't mean you shouldn't propose it there - just make sure you have a strong use case.
