Everything you wrote is correct.
Investing in currencies that are centralized always comes with risk. Exactly the same goes for Ripple.
As for why decentralized stable coins have not been as successful as USDT, I think the point is simply that the USDT has been around longer and is more popular.
The same is the case with Bitcoin, although there are coins that are more technologically advanced, but Bitcoin is still the king of cryptocurrencies. This is because it was just the first and it is more recognizable and popular. I think it's exactly the same with USDT and decentralized stable coins.