Because it doesn't appear on the financial reports, it doesn't mean that the sales have declined. What if the sales were raised more in the other themes than they were in Ninjago? Eitherway, I might not be a marketing expert, but I know for a fact that a company wouldn't get rid of a steady source of income that easily. They soft-rebooted the series because they're planning to keep the show going for much longer. Ninjago was also recently called an "Evergreen Theme" with the launch of the legacy line. Let's also not forget that other than the sets, they also have the show, two (at least) magazines, a card game, a sticker collection and much more. I understand the mentality "It's been around for so long, it has to end eventually" but realistically, I don't see it ending anytime soon.