I believe we have at least a ten years of HTML left before it's been completely phased out - mostly due to the fact that XHTML served as XML is not yet acceptable because of the way ie (doesn't) handle it, editors are still mostly outputting HTML4, and there exists a tendency for remaining with the current spec rather than changing to the newest and greatest. This is because of costs, stability, reliability and the fact most people doesn't know enough about the technology to know why they should care.
I have a firm belief that HTML will be phased out, though. There is only one stop on the way towards that at the moment, and that is ie.
So, yes, the day will come. Eventually HTML will be phased out, for good or for bad. (Yes, there ARE bad points about it.)