I do not think Sword and Shield's legs are gonna be cut off by the next game whenever that releases, at least as drastically as some are predicting it could.
There really has not been a precedent in regards to that. S/M continued to sell even when US/UM were announced, same with US/UM when Let's Go was announced. Sure, the legs will decline, but I find it hard to believe it will be that drastic. Hell, Let's Go is still selling in Japan despite having Sword and Shield just announced.