I think the hardcore crypto nuts believe Bitcoin can replace fiat currencies while I'm more moderate and believe that Bitcoin can exist in the form of a commodity or be used as a currency alongside currencies like USD. Day to day transactions aren't a stretch if merchants can deal with transaction fees and confirmation times. Network being overloaded is a concern, but presumably that's something that could be hashed out once mainstream adoption is wide enough that *everyone* is using BTC.
It will be very hard to get *everyone* using Bitcoin. I will be happy, if a simple majority of the world population approves the usage of Bitcoin. But we have a long way to go before we reach that level. Right now the user base is around 100 million, and that needs to be increased by 40x or 50x before we can claim world-wide acceptability. And as you said, scalability is a concern, but hopefully a few hard forks can resolve it.