To answer your question: yes, your current DTD is fine.
But it's also the wrong question. Standards don't work like this.
It's not about making sure you're always using the latest one, and they don't really "expire" in the way that your question seems to suggest. They're about picking one and following its rules.
Even if your site were using HTML 4 and its associated doctype, there would be no particular reason to change any of that as long as the markup is valid within the conditions of the HTML 4 spec. If you wanted to recode your site with XHTML or HTML5 or whatever that's fine, but there's absolutely nothing compelling you to do it. (Which seems to be the core of your question.) If you'll recall, there was actually a period where many were actually pushing for a return to HTML because XHTML was really never done correctly in the first place. (When XHTML was in vogue, prior to what turned into the push toward HTML5.)
There's a secondary flaw in your question in that it's not only about the doctype. If you just change that, you could actually create problems due to changing semantics or deprecated elements, etc. You need to consider entire documents. Analogically, a book set in the 80s imposes certain requirements, such as "Do not make references to Facebook." And in the other direction, you can't simply decide to move it into 2010, because you have to make sure to fix that bit where you mention that Michael Jackson just released Thriller.