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.
It will obviously go up, it's simply math and literally unavoidable. It's a matter of when, and having the balls to sell at peaks to buy back later. Insanely risk tho, you never know when a peak is going down or it will continue.