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.
It can be a good idea unless you will have $0 in your account till your retirement time because of volatility and instability of this market. And when all the prices will go down, right to the bottom.
Bitcoin and altcoins are volatile. I do not see any reason in creating the cryptos retirement account. I am more than sure that less than 1% of retired people will agree to open it.