In 3.5, liches can survive having their phylactery destroyed, no problem, as long as they're not currently using it to regenerate. This was brought up in Start of Darkness.
Way back in 1E/2E AD&D, destroying the phylactery would instantly kill the lich, but that's not how it works in the OotS-verse. I'm not sure if Xykon would even know if his phylactery was destroyed if it was done out of his sight.