If you want to sell me on the idea that tieflings are "just people", then make them a subrace of humans, otherwise I'm not interested in continuing this discussion.
Well, strictly speaking, tieflings from the fourth edition have become a subrace of humans. So, Bael Turath was a human empire, from where virtually all 4th edition tieflings originated. Nothing has changed in the fifth edition, where tieflings have human proportions, and non-human tieflings are not mentioned at all. Funnily enough, Pathfinder, who originally gave many pedigrees of tieflings and assumed that they could not only be humans, changed this in the second edition, making tieflings a subrace of humans. The same is true of aasimar.
