In 20 years you could probably have option to receive your income in Bitcoins instead so no need to buy Bitcoins anymore and pay the exchange fees. But only if the Bitcoin price doesnt move +-25% in one month regularly, which is possible for Bitcoin to get much higher exchange volumes thus stability in 20 years than today.
I think for 20 years bitcoin will continue to fluctuate but over time each year. of course users will increase with the request itself. and every year bitcoin will increasingly stable. but it took more than 20 years?
so it is quite difficult for bitcoin to become stable in the 20 years to come