This really is a tired argument. The answer depends upon how you define serialism. Personally, I like Milton Babbitt's definition:
'… a serial relation is one which induces on a collection of objects a strict, simple
ordering; that is, an order relation which is irreflexive, nonsymmetric, transitive, and
connected over the collection. The term ‘serial’ designates nothing with regard to
the number of elements, or the operations—if any—applicable to the elements or the
relations among them. A musical work, then, can be described as serial with regard
to, say pitch, if the pitch content is most completely and most simply characterized
as fulfilling such an ordering with regard to temporal and/or spatial precedence.’
By this definition, you could argue that it isn't really dead. Granted, you're unlikely to find a contemporary composition that strictly follows some sort of serial organisation from beginning to end, but you will find many localised instances of serial ordering within a contemporary composition. I suppose you could argue that Thomas Adès' use of interlocking interval cycles is an example of serial ordering. A simple canon could even be classed as an example of serial ordering. In fact, you could even argue that an isorhythmic motet from the 14th century is a serial composition. Bell changes are a form of serial ordering. Early minimalism is often serial, in the sense that it relies on process. Some spectral music is serial in a sense, as musical decisions are often dictated by the data from a Fast Fourier Transform. In my opinion, serialism has always and will always be used by composers of all styles of music in some form or other, whether they're aware of it or not. It's not dead or alive; it's just a set of tools that have been absorbed into a larger musical lexicon.
I suspect that the real question is this: is the musical aesthetic that is most commonly associated with serialism, i.e. music created roughly between 1920-1960 that uses serial organisation as a means of creating a sort of "anti-tonality" that explicitly seeks to avoid any form of tonal reference, dead?
For most people, the answer to that is probably yes. Though you can't ignore the fact that there are a lot of composers still around who are about 60+ who still adhere to this aesthetic. They may not be "important" but they certainly exist.