It is true, if you are saving your retirement funds in fiat, then by the time you retire, the money would be worth nothing and inflation would have eaten almost all of its value. However, only 'few' people would ever own 20 BTC or its equivalent in any fiat currency, only rich people can afford that, and then they would not have to save for their retirement out of their ears.

I believe retirement funds are better if put into medium risk investments, it is good to have some of it in liquid cash, but keeping all of it in cash is too 'risky' in itself, because of inflation, which will always continue rising.