Bitcoin is not able to replace fiat, at least now. I believe that there will be another cryptocurrency which would be able to replace traditional money because Bitcoin has a lot of issues as well. BTC is good, but it is too far from perfect.
Btw, I totally agree that Bitcoin is not an investment. It was created for other goals. Greedy people don't get that

I believe that it shouldn't because mining bitcoin requires more energy while other cryptocurrency can do it efficiently.
I don't think Bitcoin will replace Fiat. Because Bitcoin wasn't made to replace any currency. but to provide alternatives for the people. It is innovating our Economy and Financial stability. so replacing the Fiat defeats the sole purpose of Bitcoin at all.
For me, bitcoin shouldn't replace any flat but rather a new currency for online purposes. Its a better alternative in transaction online compared to paypal and other same services.