Just look at the Tiefling exponential. If Tiefs always breed true, even with other races, like the lore alleges they do, doesn't that mean there is an ever-expanding population of Tieflings that will eventually become the most populous, and then only, race? Except they won't because WoTC didn't think about that when they retconned Tief lore.
I hate what they did with tieflings. I mean, I like their default look in 5e; the problem is that it's also pretty much their
only look. Before the retcon they were really diverse and it was one of their coolest features. One could look like a cambion and another just have eyes of unnatural colour hinting at his fiendish heritage. It also made sense, because a quater-devil usually looked more like the former and sixth or so generation likely retained only subtle traits.
