>>36307141I don't think that Let's Go exists for this reason.
With Let's Go, they can easily produce new Kanto-based merchandise (with minimal effort, not having to create anything completely new), continue the anime (since, at this rate, Sun and Moon will end in November) etc. etc.
They needed to cover 2018 with something that could lead to that.
In 2015 they didn't publish any main game, but they continued the anime with XYZ. This cannot be done with Sun and Moon, since the arrival of Ultra Necrozma is almost imminent (the episode that will air on September 6 in Japan, in fact, is called "Prism of Light and Darkness, Its Name is Necrozma!!"), and Ash has only to face his last Grand Trial.
