They are not discontinuing support on other chains. Tether isn’t even launching this new blockchain themselves. They are only investors in new L1s that use their token as the native asset. It is not in their best interest to be exclusive to a single chain when they are so dominant across many chains.
USDC is also very dominant, especially in DeFi. It would break too many things for Circle to pull out all that liquidity. In fact, they are actually expanding to more chains as Hyperliquid will soon get native USDC.
If Tether and Circle are interested in low-cost, high-performance transactions while preserving security, launching a L2 network on top of Ethereum would be the way to go. No need to create an independent chain of their own. It seems they're more into control, power, and manipulation than anything else. Even if they stop supporting stablecoin variants on other chains, there's nothing stopping developers from creating a bridge than would link tokens from their chain to others. And if Tether and Circle's new chains are EVM-compatible, the process would be easier.
I'm yet to see how regulators would react with such a move. If they didn't allow Facebook to proceed with the Libra/Diem project, what makes you think they will allow stablecoins to have their own chain? By doing so, Tether and Circle would become centralized financial institutions (more like banks). All subject to FinCEN laws in the US and whatnot. I'd "stick" with truly-decentralized stablecoins (eg: DAI) just to be safe. I mean, they're also risky but at least they're not subject to the likes of corporate entities. We'll see what happens in the long run.