I'm going to go way out on a limb here and say that no matter what else changes, there will still be evil monsters as part of D&D. I know, radical.
Sure. Nothing wrong with having Always Evil enemies. The problem is when you pick natural, mundane creatures, like orcs, instead of picking things like fiends (which are made of literally pure, concentrated evil), constructs (which are programmed), undead (which are either mindless or tainted by evil energy), or fey or aberrations (which can have such alien mindsets that it might be impossible to to understand motivations and so you can only judge by results). Or instead of picking things like bandits, cultists of evil gods, slavers, and the like, which are mundane creatures who are actively choosing to harm other people for their own benefit.
In other words: Orcs should not be "always evil" or even "evil by default, some exceptions may apply." It should be Orc bandits, cultists, and slavers are evil, other orcs are just orc people doing orc people things.
Could you add some redeeming quality to each and every monster in the MM? I suppose you could. I don't see the point. The entries in the MM are the protagonists in the game. Are all orcs evil? Up to the DM and the setting. All trolls? Troglodytes? Meazels? Are you going to rewrite vampires so that some sparkle in the sunlight?
<sigh> This argument again?
OK, first, you have to look at the base lore of the monster and add in things like feeding and reproductive habits. Vampires historically feed on the humanoid blood, mind-controlling people so that they are "willing" to allow this. They reproduce by corrupting living people, turning them into undead spawn under their control. Vampires are, and have been for a very long time, a metaphor for rape. It may be possible to have vampires who work to non-evil ends, but their base needs require evil acts. Even if you had people actually willing to give up blood for a vampire, it would be difficult or impossible to tell if that person was truly willing or had been manipulated/brainwashed into doing this. (I would imagine you agree that nonmagical manipulation can be just as evil as magical manipulation)
You can take those two metrics--reproduction and feeding--and apply them to most creatures to help determine alignment. Troglodytes, orcs, kobolds, mostly reproduce and feed like humans do. So they might be evil--or they might not be evil.
And, well, as someone else said, chicken or egg? What came first, the alignment or the lore? I would imagine that the alignment came first (so you can kill them without
feeling bad changing alignment and losing XP for doing so) then later on lore was built to justify that alignment. And that probably people didn't want to deal with having to figure out if a group of orcs was evil or not. Like, if your party came across a group of humans sitting around a campfire, you'd have to interact with them before you could determine their alignment. But if you came across a group of
orcs doing the same thing, you can just kill them and take their stuff, plus free campfire.
So if it were up to me, I wouldn't find a redeeming quality, I'd make one. For trolls, they're very hungry carnivores and don't really think about anything else than eating. "Hungry" is neither evil nor good. It's possible to direct a troll into acts that are beneficial, just like you can direct one into acts that are malicious. Now, it's very possible that a troll mutate might become violent, cruel, and destructive, but that's like a dog going rabid. The mutation affected its brain in a bad way. Unless you have some sort of cure--which might be possible in a magical world!--you would probably have to put it down. (Of course, it's also positive that the mutation made it ultra-good/helpful. In the webcomic Freefall, some of the rebellious robots joined up in roving gangs, where they wheeled around town and committed acts of random construction. Humans were waking up to find (non-carnivorous) gazebos in their yards and that their cars had been fixed.)
How about instead they just make it clearer that alignment is just a default, that these descriptions are for the ones that the PCs are likely to face as protagonists?
What if they decided that humans were evil as a default?