No, for two obvious reasons:

--while it is not listed as a limitation on the template, demiliches do not talk. Can you imagine Xykon voluntarily assuming a form where he would never speak again?

--while demiliches can fly (and do so very quickly and with perfect maneuverability, at least in 3.5), they spend the vast majority of their time sitting around (and again, in complete silence). Can anyone picture Xykon willingly entering such a state? He'd be bored out of his skull (pun intended) in an incredibly short period of time.

Also, Xykon does do some research, but he's not the type to sit around for decades contemplating where his navel once was, or the mysteries of the universe. He's one for staying active to avoid boredom, and that--by traditional interpretations of the demilich--will stave off the decay that sends a lich toward demilichdom.