Stable coins are the hard currency in the crypt world. Professional traders would not even trade altcoins if there were no pair of USDT, USDC, etc.
Stablecoins have existed on the cryptocurrency market for several years, while altcoins have existed for much longer and earlier traders managed to trade with them. Of course, with the advent of stablecoins, trading has become much easier.
It must be admitted that altcoins are developing and improving much faster than bitcoin, which means that after some time bitcoin will have to give way to its place as the leader of the cryptocurrency. It's only a matter of time.