I don't want to get into another long debate about the pros and cons of switching, which is where this is heading. Been there, done that! "Why are the Shawnee not allowed to coexist alongside the US?", "why force America, a loaded concept of a civilization, on a player who wants to continue playing as a native American nation?"; I understand that many people are not happy about it, but that's how Civ VII works, what is left to be said?
The original point that I wanted to counter was this idea that we shouldn't "send the message that indigenous people are inevitably going extinct at the hands of European colonizers".
I simply contend that this isn't the message, since each civ game is a unique alternate history with its own narrative. Sure, everyone knows what America are based on, but you write a new history for America every time you play a game, and isn't that why we play at all? Suspension of disbelief is bread and butter for Civ.