If you are retiring in 15 years or more its well possible that 1 bitcoin can be enough to retire on even in a first world country. I wouldn't put all my eggs in one basket though and rely solely on that, I would still look at other options just incase.
Don't settle for a low amount of bitcoin aim higher, i think 10 to 20 bitcoins will be enough after 15 years or more to build an average modern looking house because the price of bitcoin is keep on growing because of the demand that keeps on coming from the new adoptions from different people and also from the countries that are getting into bitcoin because of the potential of its financial system.
Exactly! No one can possibly feel safe with 1 Bitcoin in their wallet unless they live in such a poor country where housing and living in general constitutes nearly no expense. And even then, you don't know how that may change in the next 15-20 years so one could be in for a very rude awakening...