Every month, your employer transfers Bitcoins into a time-locked address that can't be transferred out before you reach retirement age. You have the key to the address. You can verify the transfer on the blockchain.
No middle-men. No funny business. No brokers with pyramid/eyeball shaped logos. No "oops we went bankrupt and lost your retirement." No bail-ins. No raising the age of retirement.
Just Bitcoin.
We can use bitcoin now asset or a investment but we can't use it as retirement account. Being retired people is the more you need money for your needs everyday. We can use bitcoin to make our money more profitable but not just a account that you will keep money of your retirement.
However, retired people can simply start to work with the cryptocurrencies, and even make much money exactly on Bitcoin. They have much time, and they do not have to go to the work.