It's unlikely that governments would intentionally allow altcoins to devalue central tradefi. There are a few scenarios where altcoins could potentially challenge the dominance of fiat currencies:
Cryptocurrency adoption: Widespread adoption of cryptocurrencies could lead to a shift in the global monetary system, potentially reducing the importance of fiat currencies.
Central bank digital currencies (CBDCs): Some central banks are exploring the development of their own digital currencies, which could potentially coexist with altcoins and reduce the influence of traditional currencies.
Decentralized finance (DeFi): The growth of DeFi platforms, which offer lending, borrowing, and other financial services using cryptocurrencies, could potentially disrupt traditional financial systems and challenge the role of fiat currencies.