Bitcoin is still suitable for small payments, there's the lightning network and even if you use bitcoin, the fees are not that really high this time.
That's why El Salvador has adopted it as a legal tender and they're seeing the good side of it not just as a payment but as well as a store of value.
Even with the low fees and fast speeds, neither Bitcoin nor Ethereum will replace Fiat because they're decentralized. Governments want something they can control, so it's unlikely Fiat will disappear anytime soon. On the contrary, Fiat will become "digitized" giving governments and central banks greater control over people's lives.
What matters is both cryptocurrencies stay true to its roots by being decentralized cryptocurrencies anyone can use worldwide. With decentralization comes censorship-resistance. It's no wonder why governments have been unable to destroy Bitcoin and Ethereum to this date. I'm fine with crypto being an alternative to Fiat than a replacement. People will ultimately choose which type of currency they want to perform their transactions. Just my opinion
