Each campaign can, and in many cases should, have their own lore. That doesn't mean that every DM should have to make that lore from scratch.
Alignment is just one small piece of the story for most monsters, but if you take it out and then still describe them as evil have you really changed anything? If you take away the default lore what do you have left?
Have we changed anything? Yes. You've made
specific orcs evil. Just like the game makes
specific humans evil. Not because of who they are, but because of what they do.
And if you are a DM come 6E who thinks having to say to yourself as you're getting ready to start a campaign "You know... I think in this setting all my orcs are going to be naturally evil, despite what the book says" is some kind of
hardship... you obviously have so little time on your hands you probably shouldn't be making your own setting in the first place. I mean, you've already said you've made your choice on the matter. If 6E takes the "all orcs are evil" standard away, are you going to have to really put in long hours and great hardship to make your "own lore from scratch"?
Methinks you exaggerate for effect when you insinuate a DM having to make their own lore in regards to monster alignment is going to be a big problem, so better not change it.
