A fair amount of the 153 million people who bought a DS prefer those "limited" games that can be found in the mobile market. Unless the Nintendo Switch sells over 150 million units, the mobile market did take that chunk of people away from handheld, and they aren't coming back.
Excellent point! The Brain Age, Nintendogs, and Cooking Mama type games are what catapulted the DS to that insanely high lifetime sales figure. Sure the Pokemons, Marios, etc; assisted in that but a huge chunk of DS core demo moved on to Angry Birds, Candy Crush, Pet Rescue, Bakery Story etc;
Which is why I always feel it's a little unfair to compare the DS and 3DS sales figures. 3DS sales figures arent too far off GBA and hasn't had the luxury of more than a decade on the market with a phenomenon (Pokemon) occur deep into its life cycle to help catapult sales like the GB.