one of my opinion this is,
maybe after 20 years if the bitcoin will be successful until that day there's a big change that everyone are bitcoin oriented and everyone are using bitcoin, if that's the case, new technology will be made and you need bitcoin to use it, maybe that day almost everything could be bought by the use of bitcoin, transport fee, foods, electricity fee, rent fee, water fee and alot more, the transaction process will be like how we transact bitcoin today, smooth and very fast.
Bitcoin hit $1,000,000: i will sell every bitcoin i have
Bitcoin hit $1,000 again: i will sell every bitcoin i have also
Bitcoin stayed where it was: will hold it until price goes very high
Bitcoin became worthless, or a few cents: if this happens you already lose, so i will still keep my bitcoin and pray that someday price will go up again.
And which of these do you think is most probable? maybe bitcoin hit 1,000$ or bitcoin will became worthless because who knows a new and better coin will be release in the future
But if the bitcoin will be everywhere and everyone will use bitcoin for their purchases and everything will be available with bitcoin then why will one feel to sell his/her bitcoins? I think at that time we will not need to rely on other currencies.