There are two different things IMO. One is a productive asset the other is a currency.

Having both is ok. The best way to think of it.

Companies you invest in under your 401K could switch to bitcoin at any point. Your share in that company will be switched to a Bitcoin equivalent. If that makes sense.

For example you have 10 shares of Apple stock. Now worth $2000, and pay 1.5% dividend. If they switch to Bitcoin in 20 years, your unit of account will switch accordingly.

At least that’s how I see it.