I think calling optimistic to someone expecting the bitcoin price to reach 500,000 dollars is an understatement, most of the world economy will need to move through bitcoin if bitcoin is ever going to reach that price, and even if I am optimistic about bitcoin, that is not going to happen anytime soon, and if it happened, at that time its value in fiat will be irrelevant since bitcoin will be the point of reference at that point in time.
Aside from the fact that $500k per coin is a delusional prediction at this point, it basically doesn't matter what high price Bitcoin manages to reach in the future, fiat will never become irrelevant.
If we have to look at entities with the financial buying power to move Bitcoin towards unseen levels, institutions without a doubt will top the list.
If they eventually end up allocating a certain percentage of their portfolio to Bitcoin, then prepare youself for a complete market makeover ~ Bitcoin will become a plaything for them.
Fiat will never become irrelevant because it is legal tender and stores are required to accept payment in the national currency.
And yes the only realistic way for bitcoin to move up in price is if hedge funds, pension funds, banks and other financial institutions buy up a bunch and that is why having a bitcoin ETF is so important to boost its price.