But you always can forge your own narrative. We even sometimes imagine that the Old World survived the EoT and fight over the ruined Lustria to reclaim it, while summoned Seraphon are just teleported from Temple Spaceships on orbit.
And when I entered AoS, I instantrly decided that my seraphon will be meterialized ones in the forests of Ghur fighiting Orruks (and I bet, I wasn't the the only one who went that way). And, lol, GW decides to do the same thing in 1.5 years. Not so permament in the end, huh?
Such things are way harder to pull off with rules. You may homerule with your buddies as much as you want, but even then you may not be sure if you haven't gone too far with buffs. But when you want to have a game with random person or to participate in a tournament, you are stuck. And no matter, how much we can state that winning is not important, starting the game, knowing that you will most likely lose, is frustrating. You pack your modles in the morining, go for 40 minutes trip to reach your club quarters, unpack everything, getting wrecked in an one hour game and have to pack again.
Also, dropping from being top tier army is way more likely than getting brought up from the bottom. Top tier armies tend to frustrate people and get patched with a quick FAQ (yes, half of a year FAQ is a relatively quick FAQ). New battletome can become a hard counter to a new battletome or just be a new level of broken. That's not the case with a bottom tier army. They tend to be quickly forgotten because they bother no one and no one talk about them. New battletomes cannot make them better all of a sudden - only make situation worse.
So I'd better get stuck with bad lore that I can easily fix myself than with bad rules which lead to bad game experience.