he can absolutely go back to those sick beats he made back in the day no problem.
he simply doesnt want to. he grew bored of that shit really quick. he didnt even enjoy himself making those classic joints. when he finally got a chance to do what he really wanted he turned into bobby digital and never looked back. all the early classic stuff was done out of necessity. him and his friends trying to get off the streets and Rza making the illest beats he could. even the W, which was an awesome return to the classic sound, was something he made because he felt he had to.
the Rza we have now is who Rza wanted to be his entire career. i dont like his music anymore, but dude is happier as an artist than he's ever been so i respect that.
so basically he fell off because he became disinterested and wanted to do something else.