Honestly, I think many of you are looking at this the wrong way - this is a great mechanic that makes a ton of sense, and you're just getting caught up in the language they've chosen to use in implementing it. Let me explain using the example they gave us...
You start the game as the Egyptian civilization. As the era changes, you are given a choice of two paths (that we've seen so far) you can go down - the Songhai (river based civ) or the Mongols (a horse based civ). You are given a choice to shape your civ based on what the map looks like, which is awesome. Let's say after exploring in the first era you learn that there are several horse tiles nearby but only one short river. In this case, it would make sense historically for your people to gravitate towards being a horse-based civilization rather than become dependent on rivers, so you choose to evolve your civilization into a nomadic horse-based people. In living this life, it makes sense that your civ would probably develop similar characteristics that the historic Mongol people did - they'd probably develop similar traits, similar military units, and similar buildings. If the game just called this choice of path "Nomadic Horseriders", but still referred to your civ as the Egyptians, I'm guessing most people wouldn't have any problem with this mechanic at all. Rather than say your Egyptian civ has taken a "Nomadic Horseriding path (similar to the Mongols)" with a "unique mounted ranged unit (similar to the Mongol Keshik) and a "unique trait centered around enslaving enemy units (similar to the Mongols)", Firaxis just said "hey, let's just call this path the Mongols". I understand why they did it that way - because if they didn't, we'd see tons of fans complaining about "where is civ X?!?!?!" - but because they did, it opens up the argument we're seeing a lot of, which is "OMG, my Egyptians became Mongols, that didn't happen in history". Don't get hung up on the name!
I think it's exciting that we're going to be able to shape our civ to the map in order to play what's given to us. I'm sure many of you have rolled games where the map you generated was completely at odds with whatever "historic" bonuses your civ had. It makes zero sense for my land-locked Phoenician settlers to have a bunch of bonuses based around being on the coast, or for my Roman settlers to have mastery over iron weapons despite not having any iron remotely close to my lands, or for my Aztec settlers on a continent all by themselves to be fierce warrior geared around battle, whether it fits how they were in history or not. Now you can take that landlocked Phoenician civilization and develop them towards taking advantage of all the hills surrounding them, or you can develop your iron-less Romans to specialize in horseback warfare instead, or you can develop your isolated Aztecs to right haiku to pass the time. The civ you develop and grow is really going to be *your civ*, not something steered into playing any specific way. I think that's really cool.
If you're getting upset about this there's a pretty easy workaround. Anywhere you see a specific civilization mentioned in gameplay footage, just mentally put "-like" after the name of it. Your Egyptians aren't changing into Mongols, they're changing to be "Mongol-like". They're still Egyptians if you want them to be, it's your game, call them what you want. They're just developing in unique ways based on the surrounding geography - just like throughout history. We're essentially playing a game with custom civs, where real civ names were shoehorned in to meet fan expectations.