Tether (USDT) faces regulatory scrutiny, notably from SEC guidelines and EU MiCA rules, potentially threatening its 75% stablecoin market dominance. A new compliant stablecoin or USDT delisting could shift liquidity to USDC or RLUSD, causing short-term altcoin volatility (5-10% price swings). Altcoins may benefit long-term from increased regulatory clarity, boosting adoption. However, a USDT collapse could trigger a market-wide sell-off, impacting altcoins heavily. Monitor exchange liquidity and USDT reserve transparency.