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.
If bitcoin is officially regulated and well accepted like any other foreign currency, then the adoption will explode. However it is also possible that it will never gain a clear status from the government (Due to anonymity and no issuer), thus there will be less and less new people adopt bitcoin (Those who are geeky and brave enough to ignore the government directive are almost all on board now). And that will make the user base grow very slow. If the user base do not increase quickly, then 100% return in 4 years is already a quite good number