Inflation of fiat money is exactly the reason you should use bitcoin as retirement savings, since it is anti-inflation, or even better, deflation. Its purchasing power would increase long term wise. A 100% increase in 4 years time frame seems to be guaranteed due to reward halving every 4 years
I would be disappointed if Bitcoin just managed to hold its purchasing power or increases marginally in the long term. I would put aside a small amount of money, be willing to take a hit if it disappears, but also expect a substantial return if it takes off. With large scale adoption, Bitcoin's price can take off and we as reasonably early adopters, may be rewarded.