I don't really think the dex cut is such a big deal... It makes no sense to play the new pokémon games and then use the past games pokémon in your team. It's supposed to be a new experience. They wanted to focus on Unova, which makes a lot of sense.
People don't use their old teams on their new teams; they still go for the fresh experience. Besides, most games don't allow you to transfer right from the get-go and make you wait until much later. The problem is that they can't experience the novelty of uniting the teams afterwards. To have all their favorite and most sentimental Pokemon in the same place at the same time. The only reason the dex cut happened is because Game Freak refuses to delay the game or higher more staff to animate all their models, even though it would legitimately as easy as using their Sun and Moon models, since that's basically what they did for most of the returning Pokemon.