There were signs of Ninjago going in this direction in 5 and 6, but yeah, Ninjago's definitely been going in more of a "theme-of-the-week" direction since after SotFs. If they have a theme that's already successful, why risk money on an unproven new theme when they can just use that idea to keep the old theme going? An apocalyptic theme, an old west theme, a Steampunk theme, a space theme? That's four potential Ninjago waves.
Speaking of which, why don't they do two waves instead of only half a year? Like, Prime Empire is 2020 and Master of the Mountain is 2021 instead of being in the winter and summer of 2020 respectively.