To say that something is old and dying implies it was at one time alive. From what I've seen over the years code validation has never been alive. The greatest majority of sites never pay any attention to it. From the perspective I've been happy because it has given me an edge but now that Google is promoting it via the Page Speed add-on more people are aware of it and are going to make an effort to jump on the bandwagon just because Google said jump. I've always maintained it as a foundation of pride in craftsmanship and I'm content that the finished product ensures it is accessible to most devices including assisted technologies.
I really cannot stress the advantages gained from it any more than I have. My last comment on it for now though is that I consider it a misnomer to call it SEO, really it should be referred to "website optimization"