Bitcoin cannot become an alternative to fiat Currency as the man said ^- Agreed
Since Bitcoins is mostly dependent on internet , high transfer fee , lightning network is still work in progress , plus not everyone does have access to Bitcoins , nor it's is legal in most nations , therefore there is no way that it is going to actually kick out the Fiat.
being an "alternative" is different from being a "replacement" so bitcoin being an alternative to fiat doesn't mean it should "kick it out".
as for the dependance on internet, fiat is also depending on the internet. the physical cash is nearly obsolete now. if some day internet were to be cut off, the daily life stops too. so you can't use that as an excuse.
as for legality, bitcoin is already legal in 95% of the world...