"Just because the Darkling had a difficult life, doesn't mean he was right!"
He was right because he was the only one who assessed the situation realistically.
He was right because the world he inhabited was rotten to the core and needed a radical change, not a shallow refurbishment.
He was right because dismantling the Fold would not magically solve centuries of bigotry and oppression.
He was right because you can't appeal to the humanity of those who don't consider you human.
He was right because no matter how hard Grisha would try to accommodate otkazats'ya, it would never be enough because bigots didn't want Grisha nice, they wanted them dead.
He was right because another Lantsov monarch in the country that had been sucked dry by the Lantsovs in centuries is a catastrophe.
He was right because everything that happened after his death proved that there was no other wayβeverything only got worse.
He was right because he realized that genocide of his people is a more pressing issue than them having cliques in their community or them not wanting to share their tech with people who hate them.
He was right because he was the only one who made an effort to protect grisha and create a safe haven for them while numerous "saints" and other self-proclaimed "good people" were sitting on their ass.
He was right because only his actions helped Grisha in a meaningful, tangible way.
He was right because "the good guys" didn't manage to do a single useful thing while he was dead.
He was right because the story put Grisha in such a stalemate that the author didn't manage to come up with anything better than to solve racism with an absolutely idiotic dragon-shifting trick.
He was right because he was the only one who prioritized the collective survival of his people over his personal feelings and moral comfort.
He was right because he was the only character in the story in a morally defensible position and with a justifiable goal.
So yeah, even if he lived the happiest and most carefree life imaginable, he would still be fundamentally right.