HTML5 still has alt="" and still has headers=""; indeed, it defines them in much more detail than HTML4.

It's true that HTML5 won't get us all the way to a completely machine-understandable Web, but it's not clear to me that that's an important destination. I think a human-readable and human-writable Web is a much more critical destination, and that's the focus HTML5 takes. I think a much better way of getting a computer-understandable Web would be to make the computers understand the current Web. I think it's going to be much easier to get computers to understand humans than it would be to get humans to understand how to write computer-understandable content like RDF, let alone motivating most humans to actually write such content.