Keep in mind that there is a big difference between widespread usage and replacing.
I have always said that bitcoin is never going to replace fiat and it wasn't designed to do that either but I also believe that there will come a day when bitcoin can be used anywhere in the world. For example tourists would no longer need to exchange their fiat to their destination country's currency, they would just use bitcoin when travel.
Yes, I do believe also that bitcoin will not be going and never going to replace fiat currency instead it will be one of the circulating currencies all over the world provided that many countries will be going to adopt crypto as a legal tender. Actually, I don't have any idea yet what would be the value of bitcoin once it is like for example used for any transactions like pay for a set of flights but if ever this would be going to happen I know it would be a breakthrough to a crypto platform and will be going to make the price or value of the bitcoin soar high.