The idea of having Nintendo 3DS games in the Switch is something that would appeal to many people.  However, having games like Pokémon Sun and Moon, Kirby Battle Royale on the Switch would probably reduce the amount of value each game have if they are available for multiple gaming devices.  

Making Nintendo 3DS games exclusive for the Nintendo 3DS is why people buy some of the most popular titles that we know and love on the handheld device to begin with.