I wouldn't bet against MK8D reaching 20 million at least WW if it continues to be the quintessential Switch game over this holiday and quite possibly the next too. I'd have thought it would've lost some serious momentum by now considering it's still a 2014 game at its core but no. Heck MK7 is still charting as well.
20 million is the absolutely minimum.
Hell, 30 million looks more realistic.
Personally, i expect at least 25 million.