Honestly, I think that Ninjago “swallowing up” original story themes is sort of a good thing. Think about this- for master of the mountain, Ninjago set the theme around a D&D-style fantasy world, with several substantial terrain and location builds (a nice change of pace, really), and a game you could play with each set, or by combining them together. It was by all accounts very successful.
Last time Lego tried a D&D-style fantasy theme, we got Heroica. That theme was good for its time, and was a great new use of the Lego Games format. It was even produced before Ninjago became the jugernaut it is today, budget-wise. However it by all accounts failed financially, receiving 1.5 waves and having no story conclusion.
Because Master of the Mountain was a Ninjago subtheme, it got both more budget, and more attention and success, than Heroica ever could, and Lego knows this. This is why, I think, Lego’s actually done more outside-of-the-box subtheme within Ninjago than they did in the years before it (most of the beloved story-themes from the early 2000s were just reskins of older, classic themes like Adventures or Space (except Bionicle, of course, which was sort of a fluke, and Exo-Force, one theme that I will admit Ninjago has basically killed any chance of a revival for)).
Lego now knows that they can do virtually any random theme concept, slap the Ninjago name on it, and have the confidence that it will do well enough to get a great design and marketing budget.
Also, about the 4+ sets in the Legacy waves- I think these sets, while in my opinion sub-par, are totally valid. With streaming, many kids these days watching Ninjago for the first time won’t necessarily start watching from the current season, but can very easily go back and start from the begining, creating demand for these classic characters, vehicles and locations among the younger demographic.
Sorry for the text-wall.