Just a couple of things I'd like to mention.
For everyone that asked for a rework and got a champion replacement instead, you may be disappointed but you should not be surprised. We have seen to many examples of this already. The first one that I experienced was Fiora who was in a similar state as old Aatrox, a good champion in need of "quality of life" changes but not a gameplay redesign and much less a visual, voice and story redesign but that's what we got anyway. So I've said in the past be careful what you wish for...And now you know why.
Since Kayn was introduced, what Aatrox' lore described as a Darkin was at odds with the new concept of Rhaast. Before Darkin were ancient mysterious warriors, now they are sentient weapons. This is a drastic change for Aatrox, while Kayn and Rhaast were designed with this concept in mind, Aatrox was originally introduced as a legendary warrior but now that this has been scrapped, who is Aatrox' "Kayn"? I think this is what demanded a character change for the new Champion.
Dimiondo nailed it in his post though I would add to it: Old Aatrox was not a human character and by that I mean human emotion don't fit well with him. He was more of an embodied spirit, a spirit of war, blood shed and massacre. No anger, hatred or remorse are described in his lore, no reason for choosing the side of the war he backed. There is, however, a voiceover line that expresses pride IIRC "This is no mere massacre, this is my masterwork".
The last thing I would like to point out is that a reason for the changes that is constantly pointed it out doesn't hold true. Old Aatrox' was hard to balance and was unhealthy for the game. This can be said about many other champions that are constantly in patch notes as the balance team refuses to give up on them, Janna in particular seems to be hard to balance and yet her kit hasn't been replaced.
