This take is hot as groxshit - The Emperor was right about everything, except for one thing...

And that is Faith, which is inseparable part of human nature. We can't live without faith in something, or someone, be it Gods, ideologies etc.

But in everything else, he was right. He was right about the necessity to reunite Humanity. He was right about xenos. He was right about other psykers. By The Throne, his empire withstood more than ten millenia without his direct guidance and without primarchs. That speaks volumes, doesn't it?

If not for Erda and some other stupid pricks, he would have been successful in his goals and Humanity would have entered it's new, perhaps eternal Golden Age, becoming the dominant force in the galaxy that nobody can fuck with.