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.
I'm sorry, but that's a terrible idea. Bitcoin is much too volatile to be used as a retirement investment. Don't get me wrong, I love Bitcoin. But it's not the right vehicle for a retirement account.
What you see now as stable may not be as stable in the future. What's your alternative, fiat, gold? what else? The future is too uncertain for anything to be worth calling "safe retirement plan". Might as well risk becoming a millionaire with BTC.