After few years bitcoin price maybe $5000 or already replaced by other currency.
It will still depend on the price of bitcoin after 20 years. But if the bitcoin's price will reach to $5000.
Maybe you just need only 1000 btc for that years and for sure even you don't work for it. It can suffice your needs for 20 years.
I think if bitcoins price is 5000$ as people are suggesting then hardly you need 100 bitcoins for your worry free life. But I will never suggest you to sit idle and just earn with bitcoins, maybe run a new project with bitcoins with the 100 btc.