The question is not will Bitcoin be above $500000 by 2020, it is what will $1 be worth?
My guess is $1.
It will still be the world's #1 reserve currency, and still the most stable and widely used.
Worldwide economic turmoil between now and then will only result in a strengthened dollar.
Thanks for the non-answer. The question is of buying power. Saying a dollar will be worth a dollar is like calling water wet.