Logically speaking if you're pissed that you can't trade up all of your old Pokemon into the new games, it devalues the idea of catching new ones or catching some all over again for a new challenge and experience. You would be halfing or cutting 3/4ths of your experience with Sword and Shield by just catching only the Galar Pokemon and then trading all your old ones in.
If I can't transfer Pokemon into BDSP then that's unfathomably based because I can experience the satisfaction of catching all 493 over again in just two games instead of catching a few during the story and trading all the rest up.