It was revealed two days ago. Two. There is still so much we don't know. Yes, Egypt can change to Mongolia. But we don't know how many civs the game will include, how many historical routes will be available, how the transition will actually occur, whether you can toggle a game mode that only allows historical routes, how many civs it will be possible to switch to, how the crises work, how the Ages work, what happens at the beginning of each new Age, etc. etc.
You can be opposed to the idea of Egypt turning into Mongolia, all cool. It seems a bit mad to me too. But there are other sides to this mechanic that are very interesting, it has a lot of potential, not least because it gives FXS scope to add a ton of civs that we have never seen before and do them justice, owing to civs being locked to an Age.
Ultimately, if I'm not forced to turn Egypt to Mongolia, if I have other interesting options, if the pacing of the game is better than previous civ games, if the end game monotony is gone, and if, most importantly, the game is fun, then this is surely a good thing.
I have no problem with people expressing concern about the mechanic in a reasonable way, I share some of this concern. Criticism can be a good thing, when offered with good will and expressed constructively. But the amount of hyperbolic negativity and entitled conservatism that I've seen is frankly ridiculous, given how little we truly know. I will never understand this attitude to a game / series that people claim to love.