You're wrong,
You wont even need a full Bitcoin.
This is correct, if we are seriously discussing 20 or 30 years 'retirement' Either bitcoin will exist at that point and be used, have worth exchanged in various retail trades or it will be dust in the wind.
If BTC really is being used in 20 years its going to be worth a ton, 21 million vs billions of people, even if only 1% of those are using it makes that system of worth alot per coin. 1 bitcoin will be all you need, I concur :p
I agree only less than one bitcoin is enough to retire after 20 years from now. With respect to current development and acceptance of bitcoin may expect more and more development in next 20 years. So one bitcoin worth more than one million dollars. So one million dollars is more than enough as the retirement fund.