In 20 years, I think 30 BTC may be enough for retirement, if the currency continue with a good level of appreciation and adoption, of course.
I think we could not predict the future, so probably 30 BTC might not be enough.. It is better to have plenty...Bitcoin I think should not be our only retirement fund, there should always be other alternatives just in case bitcoin fails, but one thing is for sure, fiat would not fail, and other investments in real life like real estates...
yes we have no idea about that what will be the price or value of 30 bitcoin after 20 years, and what is the need of a person for spending a good life. some people spend simple life and they can compromise with a little amount of money. therefore we cannot predict about the amount of bitcoin for a person to be fulfill for his retirement after 20 years.