and the version locked exclusives in each generation are day 1 dlc

I can see how it would be seen this way, but imo the intention from the beginning wasn't that people would buy both/all three versions per generation, it was to encourage people to trade with each other. Which absolutely worked when it came out. It was a totally new idea that definitely helped sell more people on the games, because they could connect with friends and trade or battle, with later gens adding even more features for when you connected.

I could see making the dlc argument these days, but at this point you could also argue it's just out of tradition.

I can't remember ever enjoying a Pokémon game. I think it's for people who get a dopamine hit from collecting.