lets break it down shall we...
bonus AD --{Changed to}--> Adaptive Damage
AD favoritism is rampant in the game and there is no hiding it, the fact that AD Champions got a Rune that exclusively was for them was a slap to the face of all AP mains, changing it to adaptive is the equavilent of giving AP mains an Apology Caprisun to distract them from the sting of the slap, and yes i say AD champions and not Bruisers because two certain marksmen Cough Draven and MF Cough where abusining it too while Mordekaiser and Diana who are AP Bruisers (at least Diana originally was until the community forced her into the Assassin playstyle) had no use of the bonus AD
20% True Damage Conversion --{Changed to}--> 10% True Damage Conversion
i dont know how you think math works, but last i checked 10% was less than 20%, so damage definitely went down, not by much but at least riot recognized one of the problematic high damage keystones.... now we wait for the electrocute nerfs that will probably never come considering how long it took reworking conqueror.... but a man can dream
new 10% Lifesteal/Spellvamp
with the sustain keeping the users alive even longer this is indirectly nerfing damage output even more, now that more damage is needed to kill people once the sustain triggers
also finally DPS Mages/Battle Mages have a keystone option, and even then the only ones off the top of my mind that can even properly use it are Swain and Karthus...... thats 2 out of 8 battlemages...
TL;DR : this Conqueror isnt the best, but is better than the old one, and opens up rune options for champions that didnt have any good keystones before