USDT is just a stable coin, how can it compete and replace ETH and XRP? But if you are talking only about market capitalization then this is probably true as the demand for stablecoins will increase drastically in the future as more countries are promoting their use. Especially the US, they are forcing stablecoins to be pegged to the USD and promoting their use with the aim of maintaining USD dominance. That would definitely send their capitalization soaring and reaching trillions of dollars is entirely possible.

If we talk about market capitalization, it could even threaten bitcoin's top spot and I wouldn't be surprised if that happens.