The far simpler explanation is people seeking out better lives for themselves.

It's politically organised and it's against the wishes of the majority of the people. So no, it's not just a simple explanation, although it would be nice if the world was just simple.

Who is saying white people don't have the right to exist?

You said that it's vile, hateful and racist to assume immigration is making a race go extinct.
Whites saying we don't want nonwhites in our countries is us exercising our right to our own countries. You have a problem with our self-determination and wish to eradicate us through forceful immigration of non-whites against the majority of whites wishes.

Also do you alt righters have an alert system or something?

I'm not alt right. I am left wing social democratic. You assuming i'm alt right because I proclaim that whites deserve a right to self determination and a right to their own countries is very telling of your entire "totally not wanting white countries to get destroyed"-world view. You probably support BLM too. Hard to be more mask off for you at this point and you're not hiding it very well.

Please positively confirm:
"Whites have a right to their own countries"
"Whites deserve self-determination in their own countries"
"Whites deserve their own countries like everyone else"
"Whites have a right to refuse non-white immigration into their own countries"
"Mass Immigration can be used to genocide a people in an area"

If you disagree with one or more of them, then just say that you disagree with those.