Hasn't Spectralism already been around since the 1970s or so?
Yes, Grisey and Murail were the primary exponents in the '70s, however, the kinds of things they were interested in had been around much earlier. Composers such as Varese, Messiaen, even Debussy, could be considered composers who set the stage for the basic ingredients of what has been called Spectralism.
There are several composers who continue to work in the area that Grisey and Murail carved out, e.g. Iancu Dumitrescu, and his wife Ana-Maria Avram, and have expanded upon their musical expression. These days some composers are taking a very technical route, whereas earlier composers relied upon their instincts, but the primary concerns are essentially the same: using timbre and instrumental (or electronic) color to establish large form structures as opposed to harmonic, motivic development, or serial methods.
I think that the style will continue to produce adherents and new works for some time to come.