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Heirs to an ancient, infernal bloodline,
We're off to a bad start.
If you're one of the "orphans of the planes," you can't be "heir" to much of anything. A heritage is something the wealthy and powerful give their children, not something your bastard offspring can easily proclaim. Your bloodline might be ancient, or it might be recent, but as a proper tiefling, you have no real way of knowing for sure. And anyway, the chronology of your bloodline shouldn't matter -- being a tiefling is about REJECTING those that would define you by your bloodline.
tieflings have no realms of their own but instead live within human kingdoms and cities. They are descended from human nobles who bargained with dark powers, and long ago their empire subjugated half the world. But the empire was cast down into ruin, and tieflings were left to make their own way in a world that often fears and resents them.
Nobles? There is nothing noble about the rejected Other, that inverts the "classism" angle explicitly. Bargains? There is no equity here, just complicated shame. Your ancestors might have been victims or breedthralls or sacrifices. Or just people in the wrong place at the wrong time. Subjugating empires? There's never been any claim to tiefling glory except what each individual wrings out of a reluctant society.
One True History? For a race whose history shouldn't have ever been a question worth answering because even asking the question is failing to understand the race?
Centuries of other races’ distrust and outright hatred have made tieflings self-reliant and often too willing to live up to the stereotypes imposed on them. As a race without a homeland, tieflings know that they have to make their own way in the world . . .
[M]ost tieflings . . . grow up in the roughest quarters of human cities and towns. These tieflings often become swindlers, thieves, or crime lords, who carve out a niche for themselves amid the squalor of their surroundings. . . .
Some young tieflings, striving to find a place in the world, choose a name that signifies a concept and then try to embody the concept. For some, the chosen name is a noble quest. For others, it’s a grim destiny.
Hey, that doesn't invalidate PS Tieflings!
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I'm not seeing how this is at odds with an interest in the status of tieflings as outcasts, who define themselves in relation to (human) society's negative expectations of them; nor how it would get in the way of using a tiefling character to explore concepts of racism, classism, and otherization. In fact, it seems to speak exactly to all of those things.
Having a decadent noble heritage that fell due to their own hubris reads more like tieflings as a cautionary tale: there is some specific sin that if only it were corrected or atoned for, your people wouldn't be so "flawed."
PS tieflings, as orphans and castoffs, have no such noble history, no aspirational former glory, no justification for their current misery. If you are part of a marginalized group in Real World society, it is not due to some corruption on your part, or some flaw in your ancestors, it is visited upon you at birth through no blame or fault of anyone. It is the cruelty of a capricious universe. There is no excuse for handicaps and deformities and simply being born "different," there is no possible redemption because there is no sin to be redeemed from, you have done nothing wrong, but you are still the target of hatred and ire and suspicion. It is the fundamental injustice of biological inequality, of a world that isn't perfect and can't be perfect.That narrative is about overcoming
society, the prejudice and the injustice of a world that doesn't treat you as normal.
Which makes it perfect for Planescape -- the idea of the universe being flawed and in need of your characters' actions to make just is part of what makes belief such a powerful force in that setting.
Giving tieflings a decadent noble past nukes a lot of that possibility. It has blame, and it pins it somewhere specific, and allows for a "cure." It is like if Shrek ended with Fiona turning into a beautiful woman instead of a hideous ogre. It is exactly the wrong message for the kind of story that I found really compelling in tieflings before.
Which isn't to say that it's a bad story. It's just not what was interesting about tieflings. It didn't capture the fun of playing one for me. If they wanted a dark and brooding satan-spawn of a race to play the Grey Guys in their setting, they probably should've kept the whole thing the same and just changed the name to something else and we wouldn't even be having this convo.
It's a good thing if I'm going for that vibe, certainly. 
