A country always need their own currency to measure their economy strenght. I am believe fiat currency still needed even cryptocurrency mass adoption become reality. Government need fiat because its more flexible use in their own area
Fiat currency is the need of governments and when all governments in the world legalize cryptocurrency there would be no need for fiat currency because all the transactions will be made in cryptocurrency. Fiat currency as well as paper currency will be replaced by crypto. People will feel ease while using their coins directly from their blockchian wallets.