Armored Saint is John's band as much as Anthrax is Joey's. John has always sounded great in Armored Saint.
I really think that both John and Anthrax did the right thing and both will benefit from it. John continuing in Anthrax would have doomed them to a mediocre status both musically and financially for the remainder of Anthrax's time as a band. Joey coming back will, assuming the new record is solid, insure that Anthrax will regain their status as part of the Big Four.
Armored Saint will continue being a solid metal band with John fronting them. It really is the best of both worlds and the best possible outcome.