Chaosmancer
Legend
Of course they're not "valid" - they repudiate good! (But they are not fully evil.)
Gygax's alignment is not neutral on the question of whether we should be good rather than evil. It takes it for granted! (Of course a player can play an evil character. Or a LN or CN one. But that player has chosen to play a character who is not good.)
As I've already said, Planescape is incoherent.
But Gygax's PHB and DMG are not. (There are hints of Planescape in PHB Appendix IV, but it hasn't gone all the way.)
Um...
If you are going to say that any view except that which emphasizes "good" is an invalid viewpoint, then there is a massive problem. Because whether or not you think Placescape is incoherent, or that Gygax's original work was coherent, the 5e DMG and PHB present the Great Wheel and the Alignment Planes as true, as valid, and as potential philosophical outlooks. Lawful Neutral may not be a "good-aligned" position, but it is presented as a valid and viable position, and one that makes up part of what is meant by Lawful Good. Even if Gygax never intended anyone to play a non-good character, that is rather immaterial since the option has long existed, and been presented as a viable character option with a coherent worldview that is expressed to not only be possible, but objectively true on a cosmic scale.
Simply declaring "none of that was valid, it is all incoherent, we were all supposed to play heroes all along" doesn't erase what has been published for the last few decades.