Keep in mind ORAS was released in 2014, back then nintendo was just considering DLC (mario kart 8 for the Wii U was the first to get DLC and also why people kept spamming "it's ok when nintendo does it", smash followed soon) and gamefreak are slow to adapt, I believe another reason was the 3ds game's code being a monolithic mess making adding more pokemon hard. I recall people hoping they would add the ORAS megas to X/Y and being disappointed.
The ultra games had so little changes they could have been easily dlc, so between buying (and playing through) the same game with minimal changes twice I believe DLC is the least bad option, mind you I am not saying it is ideal. Ideally they should release the games in a finished state and with the full dex and the DLC should have new pokemons, forms or transformations and new areas.
Another reason they are opting for DLC is that way the mythicals/legendaries they plan to distribute later can't get leaked as soon as the demo is out/someone gets an early copy, since those won't even be in the data or at the very least have a placeholder.
My expectations for the series are low, yet legends arceus gave me a bit of hope. I read how it was made by younger staff, and considering how long people have bitched about masuda and ohmori maybe letting the new blood make games isn't so bad.
Even if legends arceus was a fluke and the mainline games continue sucking dick at the very least they are releasing spinoffs again, meaning we might get a decent game once in a while.