The Dark Eldar saved the Tau from a hive fleet for a ‘mere’ cultural exhange.
The Tau who accompanied the Dark Eldar to Commoragh were turned into Talos engines and the like, mutated horribly and tortured by Hommonculi.
The Tau have since learnt the difference between the craftworld Eldar and their debased kin. Something most Imperials don’t seem too interested in learning due to their extreme xenophobic brain washing.
The Imperium are the worst outcome for humanity.
The Interex would not have fallen to Chaos.
The fleet based societies wouldn’t have invaded and gone to war with literally all peaceful Xeno.
A sane federation of post dark age human planets could have been formed without the autocratic dictatorship and avoided war with half the galaxy.
This would have left humanity AND xenos better off and more capable of defeating emerging threats like the Tyranids.