Lets say I want to buy bitcoins today and store them for 20 years to use a my retirement fund. Live comfortably, but don't need to buy a yacht or anything. How many do you think I would need to buy?
This is a difficult question and answer it clearly can not. The fact that today bitcoin is not very stable, and to ensure not only its popularity but also the stability of the exchange rate in 20 years, no one can. Therefore, even if users are going to save, they will need time to implement.