I'll be sixty-two in 2050.
No, White people will not disappear. I don't know why people have this irrational fear of various races disappearing.
Far as other countries being in existence I can't say. But, look at it this way:
Empires rise and fall and nations come and go.
Cultures change for better or, for worst.
I'm curious to see how the racial dynamics will play out in America, especially with the growing Latino and mixed-race population. However, ain't no White folks disappearing or, anybody else for that matter.
With that being said, I do think class, gender, religion/non-religion will be more of an issue then race.
I do believe by then we'll be very close to the colonization of space, genetic engineering will be in full swing, human hoes and robotic hoes will be fighting each other over tricks, and we'll already have had a full fledge WWIII nuke-style war by then.
I think we'll be in the beginning stages of a
Ghost In The Shell,
Akira, and
Cowboy Bebop style world (Futurama is still a million years away).
At least I hope so.