Not crash but replace, there is a small chance that it will ever be replaced with another form of payment but never in my opinion will be destroyed in itself.
Yes, the paper money can't be replaced totally by the digital currency in any time because there are many processes, in which only paper money can be used and digital money is useless there. Bitcoin can replace paper money in future but upto a specific extent and it importance will continue with the importance of digital money.
Even if people started using bitcoin in daily life, then with it paper money will also play an active role. Both are having their own importance, we can't compare one thing with another by means of its importance.